Abstract: Fastgrown poplar is used widely in wood industry. But its utilization is limit especially in structure application because of its defections. It can be used after some treatments such as fireretardant,preservation, enhancement and so on. Leach resistant of retard for poplar was studied after fireretardant and enhancing treatment. In this study, the inorganic retard was used, but the treated wood had the problem during the usage, i.e. the leach of Pharmacy. It affected the effective permanence of the retard. After the nitrogen and phosphorus fire retardant composite were selected, it was studied how the fire retardants’ leach resistant was fixed by low molecule phenolformaldehyde resin under the condition of high relative humidity and the fixed effect was. The change of sizes (horizontal) and quality of the specimens were especially investigated in the experiment. The results showed that the greater the concentration of fire retardant was, the greater weight gain percentage of the samples was, and the more serious leach was in the test, after the specimen was modified with the fireretardant. It was also seen that under the same conditions (when weight gain percentage of the specimens is same), the greater the concentration of PF test solution was, the better effect of the leach resistant was. The PF resin with 25％ concentration had the best effect on reducing the leach of N-P inorganic composite retards. In conclusion from the above comparative analysis, it was advisable to improve the leach resistant of fireretardant with phenolformaldehyde resin.