1. A system for controlling scanning planes of imaging device, comprising: a tracer, on which a tracer coordinate system is built, fixed on a target which is to be scanned; a tracking device, on which a tracking device coordinate system is built, for obtaining the position and orientation of the tracer in said tracking device coordinate system and converting the known position and orientation of the target to be scanned from a coordinate system of said tracer to a coordinate system of said tracking device; and an imaging device, for scanning the position and orientation of the target to be scanned in said coordinate system of said imaging device to form images, on which an imaging device coordinate system is built; wherein, said tracking device or said imaging device has a conversion module preset therein, which is for converting the position and orientation of the target to be scanned from the tracking device coordinate system to the imaging device coordinate system; and when the conversion module is preset in the tracking device, the tracking device transfers the position and orientation of the target to be scanned in the tracking device coordinate system to the imaging device; and a way of implementation of said system, comprises step 1: building a system comprising a tracer, a tracking device, and an imaging device; and then fixing said tracer a target to be scanned; and, presetting a conversion module for conversion between a tracking device coordinate system and an imaging device coordinate system in a tracking device or a imaging device; step 2: defining a plane through which said imaging device scans a target to be scanned as a scanning plane G, and an expression of formula of said scanning plane G in said tracer coordinate system is presented as: 
 {right arrow over (n)} tool ·( X tool −S tool )=0 wherein, X tool represents a coordinate, of any point on said scanning plane G in said tracer coordinate system ‘tool’, which is 3 dimensional vector; {right arrow over (n)} tool represents a normal vector of said scanning plane G in the tracer coordinate system, which is a normalized 3 dimensional vector; S tool represents a coordinate of a known point on said scanning plane G in the tracer coordinate system which is 3 dimensional vector: step 3: measuring a position and orientation C tool track of said tracer 1 in said tracking device coordinate system ‘track’ with said tracking device 2 ; Step 4: calculating a first plane equation of scanning plane G in the tracking device coordinate system with the tracking device using the position and orientation C tool track of step 3, wherein, an expression of said first plane equation is presented as bellow: 
 {right arrow over (n)} track ·( X track −S track )=0 
 {right arrow over (n)} track =R tool track ·{right arrow over (n)} tool 
 S track =R tool track S tool +T tool track in which {right arrow over (n)} track represents a normal vector of said scanning plane G in said tracking device coordinate system, which is a normalized 3 dimensional vector; X track represents a coordinate of any point on said scanning plane G in said tracking device coordinate system, which is a 3 dimensional vector; and S track represents a coordinate of a known point on said scanning plane G in said tracking device coordinate system, which is a 3 dimensional vector; step 5: calculating a second plane equation of said scanning plane G in said imaging device coordinate system in terms of a conversion module C track scan for conversion between said tracking device coordinate system and said imaging device coordinate system, wherein, an expression of said second plane equation is presented as bellow: 
 {right arrow over (n)} scan ·( X scan −S scan )=0 
 {right arrow over (n)} scan =R track scan ·{right arrow over (n)} track =R track scan ·R tool track ·{right arrow over (n)} tool 
 S scan =R track scan ·S track +T track scan =R track scan ·( R tool track ·S tool +T tool track )+ T track scan in which {right arrow over (n)} scan can represents a normal vector of said scanning plane G in said imaging device coordinate system, which is a normalized 3 dimensional vector; X scan represent a coordinate of any point on said canning plane G in said imaging device coordinate system, which is a 3 dimensional vector; S scan represents a coordinate of a known point on said scanning plane G in the imaging device coordinate system, which is a 3 dimensional vector; and Step 6: scanning said target to be scanned with said imaging device 3 , in terms of said second plane equation calculated in step 5.