The invention relates to a switching power supply converter, which comprising a transformer including a primary winding and a secondary winding, a power switch circuit and a voltage input circuit, a voltage output circuit, an auxiliary winding, a control circuit, and a voltage sensing circuit; the control circuit performs that in a first turn-off period of the power switch circuit, acquiring a first time duration from a preset delay after the power switch circuit turning off until the sensed voltage corresponding to a preset condition, and acquiring a difference between the first time duration and a preset time duration as a second time duration; in a second turn-off period of the power switch circuit, acquiring the sensed voltage at the time point corresponding to the end of the second time duration starting from the preset delay after the power switch circuit turning off as an effective sample.