A single-mode terahertz quantum cascade laser （THz-QCL） with a two-dimensional patch antenna array as a resonant cavity is proposed and realized. The active region of each patch antenna is sandwiched between two metal layers， exhibiting full-scale subwavelength characteristics and exciting a vertical electric quadrupole mode with low radiation loss. The inter-antenna coupling within the array effectively suppresses electromagnetic leakage in the plane， allowing for a high-quality factor and low threshold current density even with only a few antennas in the array. As a result， the laser"s power consumption is reduced to 950 mW. Moreover， the discrete antenna array design provides a larger heat dissipation area compared to the heat-generating area， and with the lateral heat dissipation channels offered by the unpumped regions， the thermal resistance per unit area is as low as 5.6 mK/W/cm². By significantly reducing power consumption and enhancing heat dissipation efficiency， the laser achieves a lasing frequency of 3.18 THz， a side-mode suppression ratio （SMSR） of 19.5 dB， and a beam divergence angle of 35°×35°. It operates continuously at 3.14 mW at 20 K， with a maximum continuous operation temperature of 90 K， notably higher than that of Fabry-Pérot cavity lasers made from the same material. This work provides a novel approach to improve the continuous operating temperature of THz-QCLs.