In this paper, the authors proposed a novel structure for a-SIN:H/a-Si:H multilayer photodiode - reach-through-valley APD, analyzed two important parameters--multiplication factor M and excess noise factor Fe which mark the APD's characteristics, gave the quantized formulas for M and Fe theoretically, and discussed in detail the micromechanism of the carrier transporting in this structure. The reach-through-valley APD was fabricated successfully in laboratory. By measuring and analyzing, a new model - tunnelling was proposed to calculate M and Fe experimentally which are 5.8 and 1.48 respectively when VR=-13V.