We proposed an optical 4-ary frequency shift keying (FSK) modulation scheme applying dual-parallel Mach-Zehnder (MZ) modulator. The 4-ary FSK based on the single-side-band modulation scheme can greatly lower the transmission speed in each sub-carriers and increase the transmission performance, comparing with the 2-FSK signal. The transmission performance of the 4-ary FSK was demonstrated after a 50 km single mode fiber. The results showed that the 4-ary FSK can realize error-free transmission. Moreover, we analyzed the influence of factors (such as disperse compensation and demodulation bandwidth) on the transmission performance in this paper. The analysis of the influenced factors can provide a theoretical basic for experiment.