A new null Ronchi test method is presented to measure the shape of a large-aperture aspheric mirror. Null sinusoidal gratings are designed by using ray tracing and phase information of sinusoidal fringes. A transmission type liquid crystal display (T-LCD) screen generates null sinusoidal gratings and controls phase shift. An off-axis point source illuminates the whole mirror surface and a camera captures the phase-shifted fringe patterns containing the information of deviations of the mirror. By analyzing these fringe patterns, the actual transverse aberration and the corresponding ideal one are obtained. Then, the slopes of the deviations of the mirror are determined based on the geometric principle of the Ronchi test. The shape of the mirror is reconstructed after restoring the deviations of the mirror by integrating. Compared with the traditional null Ronchi test, the method eliminates the jagged edge of bands on the null gratings, and acquires the information of enough tested point. Simulations and a preliminary experiment validate the feasibility of this method.