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.