In recent years, low?noise femtosecond fiber lasers are widely demanded in high?precision and high?speed research such as ultra?stable photogenerated microwave and long?distance time?frequency transfer. To realize a laser seed with low?timing?jitter noise, an 100 MHz all?polarization?maintaining mode?locked Er:fiber laser was demonstrated on "optical cube". The"optical cube" configuration can effectively suppress the mechanical noise and the drift of the repetition rate. Based on the balanced optical cross?correlator technique, this is the first time to perform high?precision time jitter measurement of the pulse train directly from an all polarization?maintaining mode?locked Er:fiber laser. The integrated root?mean?square timing jitter from 10 kHz to 1 MHz offset frequency is only 98.36 as.