The capacitance-voltage (C-V) characteristics of 4H-SiC p-i-n ultraviolet(UV) photodetector with temperature and bias voltage are analyzed and compared. The deep-level defects in 4H-SiC p-i-n structure are observed. Results show that the high-frequency (1 MHz) C-V characteristics almost do not change with reverse bias due to the fact that i-layer of detector is in depletion state under near zero bias. The high-frequency junction capacitances increase as the result of the number increasing of thermally ionized free carriers with the increasing of temperature. Low-frequency (100 kHz) junction capacitances of the detector have a stronger voltage and temperature dependence than that of high-frequency junction capacitance, and the reason is that the carriers trapped by the deep-level defects are ionized with increasing of reverse bias or temperature, which affects the junction capacitance.