Measurement of planar element using slope sensor method is based on the geometry of the fringe reflection and phase measuring deflectometry method which can be used to measure aspherical shapes and transmission optical element rapidly, simply and accurately. It can be called deflectometry or Hartmann test system in reverse. We go through the derivation of measurement equation of planar element slope and wavefront reconstruction algorithm. By using an LCD screen with programmable fringe as illumination source, phase shifting method of four step is used to recognize display pixel coordinates, and a camera with small aperture images the surface under test. We apply this system to the measurement of acrylic plastic plate surface under the condition of and heating. The slope data can be obtained after operating these pictures by computer, and we use orthonormal vector polynomials fitting in a square area to calculate the surface shape of mirror under test, then analyze the surface information. And we use this system to test the slope change of acrylic plastic plate surface under the condition of stressing, and compare with the result of the laser-reflection method. The results show that the test system has a large dynamic range and is simple, rapid, non-contacting, and high-precision. It can supply a new method to test the planar surface shape and the dynamic changes of the object surface shape in practical production and application.