The addition reaction of CH2OO + H2O CH2(OH)OOH without and with X (X = H2CO3, CH3COOH and HCOOH) and H2O was studied at CCSD(T)/6-311+ G(3df,2dp)//B3LYP/6-311+G(2d,2p) level of theory. Our results show that X can catalyse CH2OO + H2O CH2(OH)OOH reaction both by increasing the number of rings, and by adding the size of the ring in which ring enlargement by COOH moiety of X inserting into CH2OOH2O is favourable one. Water-assisted CH2OO + H2O CH2(OH)OOH can occur by H2O moiety of (H2O)(2) or the whole (H2O)(2) forming cyclic structure with CH2OO, where the latter form is more favourable. Because the concentration of H2CO3 is unknown, the influence of CH3COOH, HCOOH and H2O were calculated within 0-30km altitude of the Earth's atmosphere. The results calculated within 0-5km altitude show that H2O and HCOOH have obvious effect on enhancing the rate with the enhancement factors are, respectively, 62.47%-77.26% and 0.04%-1.76%. Within 5-30km altitude, HCOOH has obvious effect on enhancing the title rate with the enhancement factor of 2.69%-98.28%. However, compared with the reaction of CH2OO + HCOOH, the rate of CH2OOH2O+ HCOOH is much slower. [GRAPHICS] .