The invention relates to a polarimeter based on differential zero-crossing detection and a measurement method. The polarimeter comprises a light source, a polarizer, a sample pipe, a prism and a photoelectric converter which are sequentially arranged from one side to the other side, wherein the photoelectric converter comprises a first photoelectric converter and a second photoelectric converter;the output ends of the first photoelectric converter and the second photoelectric converter are connected with the input end of a differential amplification circuit in a circuit processing unit; the circuit processing unit comprises a differential amplification circuit, a frequency-selecting circuit, an envelope detection circuit and an A/D conversion circuit, the frequency-selecting circuit is connected with the output end of the differential amplification circuit, the envelope detection circuit is connected with the output end of the frequency-selecting circuit, the A/D conversion circuit isconnected with the output end of the envelope detection circuit, and the output end of the circuit processing unit is connected with the input end of a processor unit; and the polarimeter further comprises a signal generation circuit, the signal output end of the signal generation circuit is connected with a light source and a controlled end of a motor respectively, and a rotor shaft of the motoris connected with the polarizer. The invention discloses the polarimeter and the measurement method aiming at the problems that a traditional polarimeter is large in power consumption, and the precision is easily interfered.