The effect of hydroxypropyl methyl cellulose （HPMC） and its content on the physical and mechanical properties of aluminate cement gypsum mortar was investigated. The results obtained by mercury intrusion porosimetry （MIP） and scanning electron microscopy （SEM） were used to make a reasonable explanation for the change of macro properties. The results show that HPMC can affect the water-retention rate， air content， setting time， flow， consistency and wet density which play a key role in increasing the plasticity and operational time of the fresh mortar. HPMC reduces the flexural strength， compressive strength， and tensile bond strength of the hardened mortar. The decrease of the strength is due to the fact that the HPMC increases the porosity and pore volume and changes the distribution of hydration products. Therefore， in practical application， the two sides of the HPMC modified aluminate cement- gypsum mortar should be considered carefully.