Cracks in autoclaved fly ash brick masonry walls are chiefly caused by the drying shrinkage of masonry during the service stage. A prediction formulation of the drying shrinkage of orthogonal masonry was derived on the basis of the mechanics of composite materials. The analysis results showed that the drying shrinkage of masonry could be approximately represented by the drying shrinkage rate of brick in masonry during the service stage, and the effect of the drying shrinkage of mortar on that of masonry was negligible. The effect of test methods, the moisture content, relative humidity, and the ratio of volume/exposed area on the drying shrinkage of autoclaved fly ash bricks was studied. It was showed that the drying shrinkage of autoclaved fly ash brick in RAPID method was smaller than that in SLOW method, and increased with the relative moisture content, decreased with the relative humidity and the ratio of volume/exposed area of brick. The prediction formulation of drying shrinkage of brick in masonry was obtained through the experiment results. The drying shrinkage of 6 masonry walls with different relative moisture contents in different relative humidity was measured, and the theoretical results was in good agreement with the test value.