The plastic free shrinkage of the mortar modified by hydroxypropylmethyl cellulose(HPMC) under accelerating drying condition were determined by laser sensors equipped with digital computer systems. The water loss of the mortar slab was weighed, and regression model of relationship between the use level of HPMC and plastic free shrinkage, the use level of HPMC and loss of water in mortar, The results indicate that the plastic free shrinkage of mortar linearly decreases with increasing HPMC content. Compared with normal cement mortar, the plastic free shrinkage of the mortar modified by HPMC of 0.1%0.4%(by mass) is reduced by 30%50%. The water loss rate linearly reduces with increasing HPMC content, the water loss rate reaches about 9%29% for the mortar with 0.1%0.4% HPMC. The fitting results show that both free shrinkage and loss of water change linearly with the use level of HPMC. Therefore, the reason of HPMC addition to reduce plastic free shrinkage of the mortar is good water retaining property of HPMC.