Abstract: The traditional complex accelerator generally contains chloride, so it is not suitable for AMPS slurry systems.Longterm strength of set cement will decline under high temperature and corrode casings.In this paper, a kind of chloride-free accelerating early strength agent complex was developed to deal with these problems.It is composed of sodium sulphate, sodium aluminate, calcium formate and calcium oxide with the mass ratio of 1:1:1:1.Experimental results show that this early strength agent is applicable to PVA and AMPS slurry systems.The strength of oil well cement can reach 3.5 MPa after 8 h at low temperature (30℃) and its thickening time is reduced to less than 80 min, initial setting time less than 90 min and initial-final setting transition time less than 10 min, so it is favorable for breakthrough prevention of oil and gas wells.This agent is successfully applied to the cementing of Ø139.7 mm production casing in Well Ban 37-15 in Dagang Oilfield, and its high-grade rate and qualification rate of cementing quality are 81.2% and 93.5%, respectively.