Based on the monthly NCEP/NCAR reanalysis data of 1950 to 2005, by using the rotated empirical orthogonal function, the leading mode of the lowerlevel Asiapacific atmosphere is extracted, putting emphasis on the interdecade variation characteristics of the coupled mode of the lowerlevel atmosphere and the relation with the interdecade variation of East Asian Summer Monsoon. It is found that there was a remarkable zonal geopotential gradient in the lower troposphere over East Asia and the subtropical Pacific. The first mode of REOF (REOF1) shows that there was a positive center at the midhigh latitudes of East Asia and a negative center in the subtropical Pacific. Corresponding to it, there was a low pressure around Mongolia and a high pressure in the subtropical Pacific during summer, their coupling variation evidently influencing East Asian Summer Monsoon. It can be seen from the time coefficients of REOF1 that there was a remarkable interdecade variation at the negative phase from 1950 to the middle 1960s, corresponding to a stronger low pressure around Mongolia and stronger high pressure over the subtropical Pacific; the coupled mode was at positive phase after the middle 1970s, corresponding to a weaker low pressure around Mongolia and a weaker high pressure over the subtropical Pacific, and at the alternate positive and negative phases from the middle 1960s to the middle 1970s. There was rather close negative correlation between REOF1 and East Asian subtropical summer monsoon, including zonal and meridional winds, especially meridional wind. The composite analyses show that when REOF1 was at the negative phase (from 1950 to the middle 1960s), there was southerly wind anomalies in the lower troposphere over the subtropics and middle latitudes of East Asia, corresponding to the stronger East Asian subtropical summer monsoon; when the REOF1 at the positive phase (from the middle 1970s to 2005), there was northerly wind anomalies in the lower troposphere over the subtropics and middle latitudes of East Asia, corresponding to the weaker East Asian subtropical summer monsoon.