A terahertz quasi-optical detector has been presented, which is mainly composed of a GaAs antenna-coupled Schottky diode chip and a highly resistive silicon lens.In order to reduce the ohmic loss, the standard terahertz Schottky diode fabrication process has been improved by forming the antenna patterns on the semi-insulating GaAs layer.Experimental responsivity and DSB conversion loss of the quasi-optical detector are 1 360 ～ 1 650 V/W and10.6 ～ 12.5 dB at 335 ～ 350 GHz range, respectively.The noise equivalent power (NEP) is estimated to be 1.65～ 2 pW/Hz1/2.Imaging experiments based on this quasi-optical detector have been carried out in both direct-and heterodyne-detection modes, successfully demonstrating its potential in terahertz imaging applications.