Due to the small size, single-molecule device may be sensitive to the ambient molecules. Thus, it is significant for fabricating single-molecule sensors to understand the influence of ambient molecule on molecular device. Based on the ab initio calculations combined with non-equilibrium Green's function method, the adsorption effects of $H_2O,$ $CO_2$ and $NO_2$ molecule on the pyrene-1,8-dithiol molecular junctions are studied systematically. The numerical results show that, the influence of $H_2O$ or $CO_2$ molecule on the pyrene-1,8-dithiol molecular junction is very slight when they are adsorbed on the pyrene-1,8-dithiol molecules, which attributes to the closed-shell ground states of these two molecules. Different from $H_2O$ and $CO_2$ molecule, being a radical, $NO_2$ molecule shows obvious influence on the electronic transport of pyrene-1,8-dithiol molecular junctions. The system with $NO_2$ adsorbate is more conductive in the positive and lower negative bias regime than those of the other two molecular systems, which is due to the evident coupling between the states of $NO_2$ molecule and pyrene-1,8-dithiol molecule.