The relative humidity（RH） and drying shrinkage（DS） of CA mortar and the relationship between them were investigated. Results indicate that RH is high in early age， decreasing with increasing in the age， and slowing down after 90 d； CA mortar shrinks after an initial expansion， the highest expansion value appear at about 7 d，and shrinkage decreases down after 120 d；the parameters of mix proportion， such as water cement ratio（mW/mC）， asphalt cement ratio（mA/mC）， has no significant influence on RH and DS of CA mortar； under sealed curing， RH of CA mortar is higher and slightly reduces in late stage， and DS of CA mortar is lower and slightly increases， even has a slight expansion in long term. Under open condition， CA mortar undergoes a high shrinkage at late stage，DS and RH show significant linear relation， which can provide guidance for reducing the crack between slab corner and CA mortar layer in CRTSⅠtrack.