Based on atmospheric circulation,outgoing longwave radiation (OLR),CMAP precipitation and sea surface temperature(SST)data,the characteristics of circulation evolution and different variable at upper and lower level over Bay of Bengal(BOB)from March to May are investigated in detail.Based on the analysis,a new onset criterion of BOB summer monsoon(BOBSM) is defined by using regionally averaged zonal wind at 850 hPa(U850)and 200 hPa(U200)over Eastern BOB(5°N15°N,90°E97.5°E).When U850 and U200 get to the criterion of U850>3 m·s~(-1) and U200<-5 m·s~(-1) and persist for five days,we define the first day as the BOBSM onset date.The criterion definition has clear synoptic significance which can reflect the stably persist variation of low-level southwest wind and the establishment of high-level southern Asian high (SAH)over the Tibetan plateau.On the basis of BOBSM onset date obstained using above criteria,the interannual characteristics of BOBSM onset and precusory singal of tropical ocean are further analyzed.The results showed that the averaged date of BOBSM onset for 19812010 is 10 th May.BOBSM onset indicate significant interannual fluctuations with onset as early as 1999 (11 th April)and the latest in 1968(1 st June).The decadal change of onset date display the trend from the late outbreak to early onset.The BOBSM onset is close related to upper and lower atmospheric circulation.In BOBSM onset early(late)years,850 hPa cross equatorial flow and westerly anomalies over tropical Indian Ocean strengthen (weaken),and 200 hPa SAH over Tibaten plateau establish early(late).The SST in the preceding winter over the Western Pacific has a good indicative sense to BOBSM onset.The monsoon onset is early(late)while SST is cold (warm).The possible impact way is to change the zonal vertical circulation and its anomalous circulation in the tropical India Ocean and the Pacific Ocean by the change of heat source,and then affect the monsoon onset early or later.