The invention provides an automatic tuning device of a transmitter loop antenna and a tuning method based on the device. The automatic tuning device comprises an adjustable capacitor array, a differential power amplifier and a tuning control device; the tuning method based on the device comprises the following steps: presetting a reference voltage; synchronously processing a differential radio frequency input signal and a differential radio frequency output signal; configuring the capacitance value of the adjustable capacitor array as a median; sampling the differential radio frequency input signal and the differential radio frequency output signal, performing IQ orthogonal frequency division and performing mixing, filtering a high frequency component in a mixing output differential signal to obtain a mixing output DC voltage, and performing look-ahead comparison and lagging comparison of the phase between the differential radio frequency input signal and the differential radio frequency output signal according to the mixing output DC voltage and the reference voltage, judging whether the loop antenna is in a resonance state, outputting a digital control instruction to the adjustable capacitance array, and adjusting the capacitance size of the adjustable capacitance array.