In order to improve the quantum efficiency of THz detectors made of p-GaAs homojunction, the effects of temperature and bias voltage were taken into account. By optimizing the materials and structure parameters of the resonant cavity enhanced p-GaAs HIWIP detectors, its quantum efficiency was increased to 17%. The relationships among the responsivity and detectivity of the detector, bias voltage, temperature and spectral frequency were simulated, leading to an optimized bias voltage range(10~40mV), an optimal temperature(< 8K) and a maximum detectivity(4.1×1010cm Hz1/2/W). By applying a pair of matched mirror, the ultimate quantum efficiency, the detectivity and the responsivity are 26%, 5.7×1010cm Hz1/2/W and 25.9 A/W, respectively.