Abstract: In this paper, the recently proposed Turbo-codes are applied to multilevel modulation, then to multistage decoding. Base on the information theory, the method of designing rates and interleaving lengths of Turbo-codes is proposed. Let the lower levels codes have higher correct error capacities, and the higher levels codes have higher transmission capacities, the transmission rate of the system is improved. The performance of Turbo multilevel codes is analyzed, and its formula of error rate is obtained. Simulation results show the validity of the design scheme and the formula.