The traditional current limiters implemented by hardware can not respond timely when facing the sudden rising of the current, while those based on the software implementation can calculate in advance the amplitude of the current according to acquisition data and respond timely before appearing of the crest and the valley. A fast forecast algorithm for current limit is introduced based on Micro Control Unit(MCU) implementation, which can fit the current acquisition data influenced by noise to the sine curve close to the ideal solution as far as possible. It can predict the current amplitude, and quickly determine whether the current crest reaches the current limit. When the circuit current increases by more than 5 times or 30 mA, it would cut off timely. Through many experiments it is verified that the algorithm can accurately predict whether the current reaches the condition of the releasing or not.