Terahertz communication with high information transmission rate and large bandwidth capacity is considered as a key technology to achieve 6G communication, which requires efficient, stable and compact terahertz sources and detectors. In order to improve the detection performance, the quasioptical terahertz detector based on integration of Resonant Tunneling Diode(RTD) and on-chip slot antenna is studied. Aiming at the problems of increased loss and parasitic effects of the circuit at high frequencies, a design criterion that jointly considers impedance match factor and antenna radiation efficiency is proposed, and a RTD detector at a small bias voltage is designed at 0.67 THz. The detection performance is characterized by co-simulating of High Frequency Structure Simulator(HFSS) and Advanced Design System(ADS). The current sensitivity of the detector at 0.67 THz is about 2.349 A/W at the input power of -30 dBm. Compared with the design scheme of maximizing impedance match factor at microwave frequencies, the sensitivity is increased by 28.01%.