An auto-calibration method is applied to a delay circuit, which includes a plurality of delay chains. If the number of accumulative errors of a designated delay chain as a current delay path is larger than a threshold value, the delay circuit scans all the delay chains and records their accumulative error numbers during a unit of time; otherwise, the current delay path is maintained. Afterwards, the number of accumulative errors is compared between all the delay chains to find out which one of the delay chains has a minimum accumulative error number, and the delay chain with a minimum accumulative error number is designated as a new current delay path.