The complex singlet potential energy surface for the reaction of CH2OH with NO2, including 14 minimum isomers and 28 transition states, is explored theoretically at the B3LYP/6-311G(d,p) and Gaussian-3 (single-point) levels. The initial association between CH2OH and NO2 is found to be the carbon-to-nitrogen approach forming an adduct HOCH2NO2 (1) with no barrier, followed by C-N bond rupture along with a concerted H-shift leading to product P-1 (CH2O + trans-HONO), which is the most abundant. Much less competitively, I can undergo the C-O bond formation along with C-N bond rupture to isomer HOCH2ONO (2), which will take subsequent cis-trans conversion and dissociation to P-2 (HOCHO + HNO), P-3 (CH2O + HNO2), and P-4 (CH2O + cis-HONO) with comparable yields. The obtained species CH2O in primary product P, is in good agreement with kinetic detection in experiment. Because the intermediate and transition state involved in the most favorable pathway all lie blow the reactants, the CH2OH + NO2 reaction is expected to be rapid, as is confirmed by experiment. These calculations indicate that the title reaction proceeds mostly through singlet pathways; less go through triplet pathways. In addition, a mechanistic comparison is made with the reactions CH3 + NO2 and CH3O + NO2. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for understanding NO2-combustion chemistry.