Differential Frequency Hopping (DFH) is a novel technique for the frequency-hopping system in HF (High Frequency) bands. The frequency transition function (FTF) and the signal detection methods are the key issues for DFH system. Symbol-by-symbol detection and sequence detection are adopted in DFH system. With these two detection methods, the symbol error rate (SER) performances of DFH system are analyzed over additive white Gaussian noise (AWGN) channels with Multi-tone Jamming (MTJ). Simulations validate the analyses. The results of analyses and simulations prove that the DFH system with sequence detection can achieve better anti-jam (AJ) performance than that of conventional frequency-hopping (FH) system.