1. A display apparatus that improves contrast by dynamically adjusting backlight thereof, comprising: a receiving circuit for receiving and processing an incoming signal to generate an image signal; an image processing circuit, coupled to the receiving circuit, for processing the image signal and generating a driving signal; a display unit, coupled to the image processing unit and driven by the driving signal to display contents of the image signal; a luminance analysis circuit, coupled to the receiving circuit, for analyzing the image signal and generating a luminance analysis result, wherein the luminance analysis circuit comprises: a luminance classifying circuit for receiving the image signal and classifying the luminance value of each pixel into a plurality of luminance levels to generate a luminance data; a calculating circuit, coupled to the luminance classifying circuit, for calculating the number of pixels of at least one of the luminance levels and an average of the luminance values of all pixels in a frame, and generating a luminance index at least comprising the number of pixels of one of the luminance level selected from all the luminance levels and the average of the luminance values of all pixels in a frame; and a luminance index analyzing circuit, coupled to the calculating circuit, for analyzing the luminance index to generate the luminance analysis result according to a table referring to both of the number of pixels of one of the luminance level selected from all the luminance levels and the average of the luminance values of all pixels in a frame at the same time; a backlight module for irradiating the display unit, comprising a first MOSFET, a second MOSFET, a voltage transformer, a lamp and a resistor, wherein the voltage transformer is coupled to the first MOSFET, the second MOSFET, a voltage source and a ground point, and the lamp is coupled between the voltage transformer and the resistor; and a backlight controller, coupled to the luminance analysis circuit and the backlight module, for generating a control signal according to the luminance analysis result to control the backlight module, wherein the first MOSFET and the second MOSFET receive the control signal at the gate terminals of the first MOSFET and the second MOSFET respectively.