The paper describes development of the detailed structure and circuit diagrams of the continuous wave NQR temperature sensor with increased conversion linearity. It is experimentally established that at amplitude modulation of 40% and change of input voltage in the range of 20–1000 mV, the circuit of a symmetric marginal oscillator with a linear active demodulator provides better linearity of transfer characteristic than the circuits of asymmetric marginal oscillators with JFET or diode detectors. As a thermometric substance of the proposed NQR sensor, copper oxide Cu2O was used, which is characterized by a strong temperature dependence of the resonance frequency of 63Cu NQR. In contrast to 35Cl NQR in KClO3, for cuprous oxide the temperature dependence of 63Сu NQR frequency in the frequency range 26.621–25.658 MHz is linear in the temperature range 100–390 K. It is experimentally confirmed that the use of a low mass sample (less than 200 mg) as a thermometric substance of the proposed NQR sensor is quite sufficient for successfully observation of the resonance line at the SNR equal to 9.1 dB.