The effects of hydroxyethyl methyl cellulose(HEMC) with high and low substitution degrees on the properties of sulphoaluminate cement mortar under fixed fluidity were studied. Test results show that water demand and setting time increase significantly while wet density and strength decrease significantly with the increase of HEMC content. Flexural strength is less influenced than compressive strength by HEMC content. Compared with the low substitution degree HEMC, high substitution degree HEMC to some extent leads to lower water demand and wet density together with lower strength and similar setting time of sulphoaluminate cement mortar. Incorporation of HEMC slows down the acceleration of sound velocity at early stage which is in favor of pot life, and leads to the decrease of final sound velocity. Moreover, the acoustic wave propagation speed is affected by the content and the degree of substitution of HEMC. The strength of HEMC modified sulphoaluminate cement mortar has good liner correlation with both sound velocity and wet density.