The present invention discloses an isolated power converter circuit and a control method thereof. The isolated power converter circuit includes: a transformer circuit, a power switch circuit, an opto-coupler circuit, and a control circuit. The transformer circuit includes a first winding and a second winding. The power switch circuit is coupled to the transformer circuit to control it according to a driving signal. The opto-coupler circuit generates a feedback signal. The control circuit is coupled to the power switch circuit and the opto-coupler circuit, for generating the driving signal according to the feedback signal. The control circuit includes a distinguishing circuit for distinguishing a status of the feedback signal.