A PRML system with a branch estimator. The PRML system includes an analog-to-digital converter (ADC) for receiving an analog input signal and converting the analog input signal into a digital sampled signal according a sampling clock; a branch estimator for receiving the digital sampled signal and estimating each branch eigenvalue; and a Viterbi decoder for decoding an output signal according to the digital sampled signal and the branch eigenvalues. Since the PRML system employs the branch estimator to estimate the branch eigenvalues of trellis of the Viterbi decoder directly, the PRML system can be simplified and the execution speed of the PRML system can be increased.