A method for checking an out-of-step of a synchronous motor includes detecting electric degrees of the synchronous motor, in which the electric degrees comprise at least a first electric degree and a second electric degree detected at a preset interval, and the second electric degree is detected after the first electric degree; comparing the first electric degree with the second electric degree to obtain a comparing result; and determining that the synchronous motor is out of step when the comparing result satisfies a preset requirement. It is determined that the synchronous motor is out of step when the electric degree keeps unchanged or decreases progressively, or an increment of the electric degree is very small.