It is greatly significant to the inheritances of ancient building relics, by improving the fire resistance of timber structures. A flame-retardant primer coating resin was prepared, then applied on spruce and radiata pine samples at the laboratory. The mechanism of the wood ignition time and combustion behavior were explored. The results showed that the flame-retardant resin has a significant effect on extending the ignition time up to 9 minutes. The rapid heat release in the initial stage of wood combustion is effectively suppressed and the peak heat release rate is significantly reduced, and thereby the risk of spreading fire caused by heat convection and transfer is greatly reduced.