The invention discloses a concrete hardening agent and a preparation method thereof, which belongs to the field of concrete additives. The concrete hardening agent contains acrylon, sulfuric acid, a tertiary amino-containing monomer, a monomer A (unsaturated carboxylic acid or/and unsaturated carboxylate), an initiator, an inorganic base and water. The preparation method of the concrete hardening agent comprises the following steps: adding weighed acrylon, sulfuric acid and water into a reactor according to parts by weight, and reacting at 86-100 DEG C for 1-5 h; adding the tertiary amino-containing monomer, the monomer A and the initiator into the reactor, reacting at 90-100 DEG C for 4-6 h to obtain a mixture B; and adding the inorganic base into the mixture B, stirring for 0.5-2 h to obtain the concrete hardening agent. For medium/low-strength level concrete, the concrete hardening agent disclosed by the invention can reduce cement content and can ensure operability and strength, and has wide application prospects.