Abstract: Cyclic prefix(CP) is very important to prevent inter-symbol interference in TD-LTE systems, the receiver must calculate the type of CP in order to obtain the right FFT decode window. Base on the CP’s character, copying fixing length data from end of the symbol sequence, presents two algorithms,sequence correlation and energy difference, which can confirm the type through correlation degree and energy difference between different sequence lengths. Finally, considering algorithm complexity and DSP realization, improve on energy difference algorithm, using part sequence instead of whole sequence. Simulation result and algorithm theory show the effectiveness and low complexity.