A structure of thermal terahertz detector working at room temperature was proposed. The detector is consisted of an on-chip antenna and a temperature sensor. The antenna made of gates of the temperature sensor absorbs incident terahertz wave and converts it to Joule heat. The heat-generated temperature rise is then detected by the temperature sensor. The working flow of the detection can be divided into three parts: electromagnetic absorption, thermal-heat conversion and thermal-electrical conversion. Modeling and simulation of every process are presented. The simulated Antenna absorptivity is 0.897, the heat transfer efficiency is 165 K/W and the thermoelectric conversion efficiency is 1.77 mV/W. The detector is designed based on 0.18 μm CMOS technology with post-process thinning the silicon substrate to 300 μm. Its simulated voltage responsivity is 262 mV/W at 3 THz frequency, while the tested value is 148.83 mV/W under the incident power of 1mW.