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 1360~1650 V/W and 10.6~12.5 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.