The influence of the InP cap layer doping density of InGaAs/InP SPAD is studied through theoretical calculation and comparative experiment. Theoretical results show that lower cap layer doping density is beneficial to suppress premature edge breakdown, reduce tunneling carrier generation rate, and increase breakdown probability. Experimental results show that devices with unintentionally doped cap layer have achieved 20% single photon detection efficiency and 1 kHz dark count rate at 223K. Compared to devices with cap layer doping density of 5E15/cm^3, the single photon detection efficiency increases by 3%～8%, and the dark count rate decreases by about an order of magnitude. It is demonstrated that reduce the cap layer doping density is beneficial to improve the performance of InGaAs/InP SPAD.