The small signal equivalent model of a Uni-Traveling Carrier Double Hetero-junction Phototransistor (UTC-DHPT) was established. In this model, the optical generated current in UTC-HPT was introduced reasonably as part of base current. The influence of single carrier transportation in UTC-DHPT on photo transconductance, emitter junction effective resistor, emitter junction capacitor, and collector junction capacitor were analyzed. Based on the established small signal equivalent model, the dependence of optical characteristic frequency and the optical generated current gain on optical window area and illumination light power of an InP-based UTC-DHPT were analyzed. At certain illumination light power, there is an optimum window area which can achieve the maximum optical characteristic frequency. Meanwhile, the optimum window area would change in certain range as illumination light power increasing. When chosen the optical injection window of 8×8 μm2, With the gradual increase of illumination light power, the characteristic frequency increases at first and then decreases, the highest value reaches 150 GHz at 280 μW. The gain of optical generated current increases gradually too and reaches the saturation gain of 82 dB under 750 μW.