1. A decoding system for eight-to-fourteen modulation (EFM), comprising: an analog to digital converter (ADC), which receives an analog signal with an EFM feature, and converts the analog signal into a digital signal with the EFM feature; an adaptive equalizer, which is connected to the ADC to convert the EFM digital signal into a first signal with a minimum phase feature; and a Viterbi decoder, which is connected to the adaptive equalizer to receive the first signal and to generate a decoding signal in accordance with a Viterbi algorithm and a channel model; wherein the Viterbi decoder performs a Viterbi decoding based on a modified Trellis which is generated by modifying a trellis for decoding two bits at a time and discards nonexistent paths in accordance with the EFM feature when computing branch metric of a branch to a node.