Continuous variable quantum key distribution (CVQKD) has high secret key rate an d other advantages,which,however,are slightly insufficient in secure transmissio n distance and discrete modulation continuous variable quantum key distribution protocol has been proved to be able to obtain better performance than gaussian modulation with low signal-to-noise ra tio (SNR).Based on this,we analyze the performance of discrete modulation two-state,four-state and eight-state prot ocols in detail when the source is untrusted, drive the secret key rate expression.Numerical simulation results show that the discrete modulation coherent state and homodyne detection with direct reconciliation outperform all other protocols whe n the source in the middle,and the eight-state protocol has a longer transmissi on distance.