To improve the responsivity and working speed of an InP-based optical detector, an uni-traveling carrier heterojunction phototransistor with gradual coupled ridge waveguide is designed. The optical transmission mode of the gradual coupled ridge waveguide is analyzed through effective index method and beam propagation method, and the waveguide width and length are optimized as 2.6 μm and 250 μm respectively to achieve single mode transmission and high absorption efficiency. Because the direction of optical transmission is perpendicular to the direction of carrier motion, the absorption efficiency and speed of the phototransistor are optimized, and the output photocurrent and characteristic frequency of the device are improved. The responsivity is 33.83 A/W, the saturation output photocurrent reaches 90 mA and the highest characteristic frequency is 87 GHz. Compared to the vertical-illuminated uni-traveling carrier heterojunction phototransistor, the saturated output current and characteristic frequency of the uni-traveling carrier heterojunction phototransistor with gradual coupled ridge waveguide are increased by 20% and 24% respectively. However, the uni-traveling carrier heterojunction phototransistor with gradual coupled ridge waveguide has a bigger absorption volume, and then the input light power at which it gets the saturation current is larger too, therefore its responsivity is slightly smaller than that of the vertical-illuminated uni-traveling carrier heterojunction phototransistor.