Quasi-optical mixers for terahertz receiver are designed and realized. The quasi-optical mixers consist of two parts: an antenna integrated Schottky diode chip and a high-resistivity silicon lens. The cutoff frequency of the Schottky diode is 3.5 THz. The planar double slot antenna, equiangular spiral antenna and balanced log-periodic antenna are integrated with the Schottky diodes respectively. The silicon lens is adopted to improve the radiation performance and reduce the surface waves. The operating frequency of quasi-optical mixers is 340 GHz. The detection capability, radiation pattern and frequency conversion are measured. Its Double Side Band(DSB) conversion loss is below 15 dB.