Dark-current-voltage curves and photon-current-voltage curves were measured by a passive quenched circuit so that the voltage applied to the avalanche photodiode can be much higher than breakdown voltage in study on the depth of punch through. The photo-current-voltage curve indicated clearly the punch-through voltage while the dark current-voltage curve is insensitive to the punch through. Furthermore, the avalanches initiated by the photo-generated carrier at a voltage lower than that from the thermo-generated carriers and explained based on the different distribution of the carriers.