The invention relates to dihydropyrimidine compounds and application thereof in medicines, particularly application in medicines for treating and preventing hepatitis B. Particularly, the invention relates to compounds disclosed as general formula (I) or (Ia), or enantiomers, diastereoisomers, tautomerides, hydrates, solvates or pharmaceutically acceptable salts thereof, wherein the variables are defined in the specification. The invention also relates to application of the compounds disclosed as general formula (I) or (Ia), or enantiomers, diastereoisomers, tautomerides, hydrates, solvates or pharmaceutically acceptable salts thereof, especially application in medicines for treating and preventing hepatitis B.