The invention relates to a high-precision harmonic wave gear transmission built-in torque sensor. The sensor comprises a DC voltage stabilizing module, a straight-angle strain rosette, a Wheatstone half-bridge circuit, a signal amplification circuit, an adder and a signal acquisition circuit. The method comprises steps that strain amount generated by a flexible wheel during transmission of a harmonic wave gear is converted by the straight-angle strain rosette pasted at the bottom portion of the flexible wheel into resistance change amount and is then converted by the Wheatstone half-bridge circuit into voltage signals, disturbance signals caused by rotation of a wave generator and the flexible wheel can be removed through signal amplification and signal superposition, load torque signals are acquired, and the load torque signals are converted by the signal acquisition module into digital quantity for output. Through the sensor and the method, disturbance in the measurement signals caused by rotation of the wave generator and the flexible wheel can be effectively inhibited, a measurement error caused by a self error of a strain gauge and a pasting error can be solved, high-precision and high-integration transmission torque measurement of the harmonic wave gear can be provided.