A testing method is performed by at least one processor and includes following operations: converting first data associated with a scan test into a program, in which the program is configured to observe an untested part of a circuitry that is unable to be tested in the scan test; performing circuit simulations with the program according to a netlist file indicating the circuitry and testing patterns, in order to rank the testing patterns to generate second data; selecting at least one candidate testing pattern from the testing patterns according to the second data; and performing at least one fault simulation on the circuitry according to the netlist file and the at least one candidate testing pattern, in order to test the circuitry.