A PRML detector suitable for high-density DVD (HD-DVD) system is developed in this paper. After comparing the spectrum characteristics of the linear optical channel model with various PR polynomials (memory length from 4 to 7)，the criteria of determining memory length and coefficients of target PR is given. The simulated SNR-BER curves show that the Viterbi detector's performance is much better when the memory length of PR is seven. The architecture of a Viterbi detector in PRML system is also described. By computer simulation and experiments，it is proved that the performance and speed of the PRML system with memory length seven can completely satisfy the requirements of the HD-DVD system.