To investigate the feasibility of using a constant thermal conductivity for fire spray coating when calculating protected steel temperature in fire, a series of fire tests on spray coating protected steel sections with a range of section factors and coating thicknesses were tested in fire. The constant thermal conductivity was calculated as the average temperature within the temperature range of interest for fire resistance design and was used to calculate the protected steel temperatures. The calculated steel temperatures were compared with the measured values from the fire tests. The results of this comparison suggest that it is feasible to use a constant thermal conductivity for spray coating. The constant thermal conductivity value would be independent on the steel section factor and dependent on the coating thickness, which indicates that small steel plates can be used as standard specimens to predict the steel temperature of other steel members protected with spray coating. Linear fitting is performed on the equivalent thermal conductivity of different coating thickness, and result of coating thickness 20mm is selected as the characteristic value of the equivalent thermal conductivity of fire spray coating.