The invention relates to an early strength polycarboxylate compound water reducing agent for a ballastless rail slab, which relates to the field of building materials. The water reducing agent is prepared from 1 to 5 weight percent of early strength component, 5 to 10 weight percent of viscosity reduction component, 1 to 50 weight percent of shrinkage component and 80 to 93 weight percent of mother liquor which are mixed together and dispersed in water, wherein the mother liquor is a polycarboxylic acid high performance water reducing agent; the early strength component comprises calcium nitrate, sodium nitrate and/or triisohydramine, and triisopropanolamine; the viscosity reduction component is polyethyleneglycol; and the shrinkage component C is fatty group polyethenoxy ether. The water reducing agent is applied to manufacturing the ballastless rail slab, can obviously reduce the viscosity of concrete, and satisfy the requirement on the fluidity in the process of concrete construction, wherein the slump is between 140 and 200 millimeters, the compressive strength within 16 hours is more than 48 MPa, and the high performance concrete for the ballastless rail slab completely satisfies the technical requirement of 'Tentative Technical Condition of High Performance Concrete for Passenger Dedicated Lines'.