In order to improve the accuracy of surface testing by long trace profiler (LTP), a LTP with wavefront equal-optical-path multiple-beam splitter is proposed. This structure can separate the incident beam into several coherent beams with equal-intensity and zero optical path difference. The position and the intensity distributions of the interferential fringe on focal plane of the Fourier transform (FT) lens are analyzed in theory. The effects of parameters of multiple-beam splitter on the width and amplitude of zero grade interferential fringe and the amplitude of secondary maximum interferential fringe are discussed. A new beam splitter based on multiple-beam interference is designed by choosing appropriate parameters, and compared with traditional beam splitter by simulation. The collimator and FT lens in the measurement system are designed. The complete optical model is established in Zemax and verified by experiment. The results show that multiple-beam interference LTP can realize the measurement of the slope of the surface under test. The width of interferential fringe on the detecting plane is smaller than traditional double-beam interference, and the intensity is more concentrated. It can improve the accuracy of the LTP measurement.