1. A magnetic resonance imaging water-fat separation method, comprising the steps of: acquiring in-phase image raw measurement data and out-of-phase image raw measurement data with an MRI device; in a computer, reconstructing an in-phase image and an out-of-phase image according to a system matrix and said raw measurement data using a penalty function regularized iterative reconstruction method; in said computer, calculating water and fat images according to said in-phase image and out-of-phase image.