Based on the RMM(Real-time Multivariate Madden-Julian oscillation) index from Australia Meteorological Bureau,the tropical atmospheric intraseasonal oscillation is divided into eight phases.The variations of typhoon genesis frequency and location over the northwestern Pacific during the eight phases are statistically analyzed.It is found that more(less) typhoons generate during the active phases 5,6 and 7(the inactive phases 1,2 and 3) over the northwestern Pacific.The box difference index(BDI) is further used to investigate the differences in environmental factors between the active and inactive phases.Results show that,compared to those during the inactive phases 1,2 and 3,the stronger low level convergence and upper level divergence outflows,the higher relative humidity in middle troposphere,as well as wider areas with small vertical shear can be found during the active phases 5,6 and 7.Moreover,the most significant difference in large scale environment field between the inactive and active phases is dynamical factor,especially low level convergence field.