Effects of air entraining agent on the primary efflorescence and secondary efflorescence of Portland cement based decorative mortar（PCDM）and its mechanism were studied. The results show that air entraining agent significantly prohibits the primary efflorescence of PCDM by its outstanding air entraining effect and procoagulant effect. Air entraining agent also significantly reduces the secondary efflorescence of PCDM with ages shorter than 7d， and eliminates the secondary efflorescence of PCDM with ages longer than 7d， so that it decreases the proportion of small capillaries， and the concentrations of K+， Na+ in the pore solution， and also prohibits these ions mobility， accordingly greatly decreases the total leachability of alkali ions on the surface of PCDM. So the solid content in the surface leaching solution and secondary efflorescence degree of PCDM are reduced.