Effect of super water absorbing polymer(SAP) on the long term drying shrinkage and autogenous shrinkage of cement mortar was investigated, and the working mechanism of SAP was discussed. The results show that SAP can greatly improve the long term drying shrinkage resistance of cement mortar under different curing humidities. With curing humidity decreasing, the improvement of SAP enhances. Besides, SAP can significantly decrease the autogenous shrinkage, especially at the early age. The dosage variation and particle size distribution of SAP greatly affect the drying shrinkage and autogenous shrinkage with the existence of the optimal dosage of SAP. But the effects of particle size distribution variation of SAP at middle and late age are generally identical. The improvement mechanism of SAP on the long term shrinkage of cement mortar is mainly attributed to the pronounced increase of the internal humidity by SAP with the slow release of absorbed water from its particles, and the optimal SAP dosage is closely related to the comprehensive effects of the improvement of internal humidity by SAP to decrease shrinkage and the collapse of swollen SAP particles to increase shrinkage.