A laser wavefront measurement technique by using a special quadratic grating is reported. The quadratic grating has a different focusing power in nonzero orders, for the grating lines are not line but arcs of circles, and multiple planes can be imaged onto a single image plane. The spacing relation between two planes imaged on a single plane was deduced for a convergent beam. The experiments of measuring convergent beam wavefront with a quadratic grating are carried out. It is found that the defocusing aberration is as large as -2.93λ and the spherical aberration is equal to 1.34λ, which are in good agreement with theoretical ones(the defocusing aberration is -2.695λ, λ supposed to be wavelength). This technology can realize real-time wavefront measurement with high spatial resolution, high phase accuracy, low cost and simple structure. Because wavefront of high-energy laser beam usually changes rapidly, this real-time technology is a useful measurement method on this application.