The invention provides a method for detecting VOC in circulating gas based on UV photolysis. The method comprises the following steps: (S1) cleaning an oxidization device by virtue of dry air; (S2) after the cleaning, starting a feeding device to introduce all to-be-detected gas into the oxidization device, and stopping the feeding device; (S3) starting a circulating device, and after the concentration of CO2 displayed by a measuring device reaches a stable state, immediately recording a temperature T1, a pressure intensity P1 and a CO2 concentration C1; (S4) turning on an ultraviolet lamp, and after the concentration of CO2 displayed by the measuring device is increased to be a maximum value, immediately recording a temperature T2, a pressure intensity P2 and a CO2 concentration C2; and (S5) calculating the molar concentration of total organic carbon in the VOC of the to-be-detected gas. The method has the beneficial effects that the temperatures, the pressure intensities and the CO2 concentrations before and after the oxidization are respectively tested, and the molar concentration of total organic carbon in the VOC can be obtained through simple calculation; and the structure is simple, the operation cost is low, and the measurement precision is high.