Mechanical properties evolution of cement mortars modified by hydroxyethyl methyl cellulose (HEMC) subjected to sulfate attack and the improvement mechanisms of HEMC were investigated. The results show that HEMC can greatly decrease flexural strength and compressive strength, but increase tensile bond strength of cement mortar subjected to sulfate attack. The short term sulfate attack is beneficial to improve mechanical properties of HEMC modified cement mortar, but long term sulfate attack significantly reduces mechanical properties. However, HEMC modified cement mortars still assume excellent mechanical properties after exposed to sulfate attack, the reason is due to that HEMC can optimize the pore structure and interfacial zones and remarkably reduces the water and ions penetration in cement mortar. Besides, as for mechanical properties under long term sulfate attack, there exists an optimal dosage range of HEMC in cement mortar.