In order to observe the coefficient of lateral earth pressure at rest of saturated expansive soil at different dry density, an improved testing device and method was used to avoid the influence of initial horizontal stress due to specimen preparation by adjusting the opening and closing degree of the ring knife and the reduction of effective area due to the compression of the specimen by using special knife. The compared tests of expansive soil form Baise in Guangxi were carried out, including with and without initial horizontal stress and variation of effective area. The results show that the initial horizontal stress due to specimen preparation can occur 7.3%~19.9% decrease in the coefficient of lateral earth pressure at rest under the same dry density, and the reduction of effective area can occur 2.3%~15.1% decrease in it under the same dry density. The coefficient of lateral earth pressure at rest of saturated expansive soil decreases as the dry density increases. The K0 of saturated expansive soil under different upper load can be obtained by the fitting formula of K₀-ρ_d.