A synchronous circuit of a receiving device of a wireless transmission system composed of a delaying unit, a multiplier, a matched filter and a checking unit is disclosed. The delaying unit receives a preamble of a baseband signal and delays the preamble that includes a plurality of symbols. Through the multiplier, the symbols of the preamble are multiplied by the symbols of the preamble delayed by the delaying unit so as to generate a plurality of matched data. Then the matched filter matches and filters the matched data to generate a plurality of output data according to a plurality of matched parameters. At last, the checking unit checks these output data to obtain the maximum output data and the corresponding symbol so as to generate a synchronous signal. Therefore, the receiving device demodulates the baseband signal and obtains the data correctly.