The invention discloses a switch power supply and a feedforward compensation circuit. The feedforward compensation circuit comprises a negative voltage identification module, a current mirror image module and a current superposition module, wherein the negative voltage identification module is used for carrying out negative voltage identification on the voltage of the feedback base pin of the switch power supply according to the secondary degaussing time of the switch power supply so as to obtain a feedback current corresponding to the primary input line voltage of the switch power supply; thecurrent mirror image module is used for generating a compensation current through a current mirror image according to the feedback current, and limiting the compensation current according to the conduction time of a power switch tube in the switch power supply so as to acquire the limited compensation current; and the current superposition module is used for superposing the limited compensation current to the over-current detection base pin of the switch power supply so as to carry out feedforward compensation on the current of the power switch tube. Regardless of whether the switch power supply is in a light load state or a heavy load state, an appropriate current compensation amount can be acquired, and the switch power supply is guaranteed to output a constant current.