The predissociation dynamics of B Rydberg state of methyl iodide has been studied with femtosecond two-color pump-probe time-of-flight spectra at pump pulse of 400nm and probe pulse 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be fitted by biexponential decays with time constants of O 1 and O 2, O 1 was assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and O 2 reflects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.