A 1.2-GHz gated infrared single-photon detector based on InGaAs/InP avalanche photodiode (APD) is designed. The APD is working in Geiger mode, gated by 1.2-GHz pseudo-sine wave signal, cooled by a 4-stage Peltier cooler with fancooling. A group of simple 9<sup>th</sup> order Bessel LC low-pass filters are used to suppress the transient response of the APD by 60 dB. The typical detection efficiency, dark-count probability and afterpulse probability of the detector were 15.1%, 3.76×10<sup>-6 </sup>/gate and 1.26%, respectively. The detector is based on commercially available inexpensive devices and can be manufactured easily.