Despite its successful application in controlling excessive water production in many mature oilfields, polymer gel is facing some application limitations under harsh reservoir conditions. To settle these problems, an environmental friendly improved inorganic aluminum gel that composed of polyaluminum chloride (PAC) as main agent, urea as activator, and sodium sulfate as syneresis inhibitor was developed. The effects of mass ratios of PAC and urea, component concentrations and temperature on gelation performance were studied. The gel compatibility with various formation brines, long-term thermal stability, and permeability reduction ability were evaluated to account for the feasibility of gel application. Results showed that as the mass ratio of PAC and urea increased, the gelation time increased and the degree of syneresis decreased. The gelation time and the degree of syneresis decreased with the increase of sodium sulfate concentration, which indicated that sodium sulfate could play a role in accelerating gelation and inhibiting gel syneresis. The gelation time decreased with increasing temperature. The gel could tolerate sodium chloride concentration up to 150 g·L−1 and calcium chloride concentration up to 25 g·L−1. After aging for 120 days at 130 °C, no syneresis was observed in gel samples, which indicated that the gel had good, long-term thermal stability. The gel had good permeability reduction ability and was effective in plugging high permeability zone. Thus, these results indicated that the improved inorganic gel could be a potential candidate for conformance control under harsh reservoir conditions.