We present a direct ab initio dynamics study on the hydrogen abstraction reaction CH2O + HO2 -> CHO + H2O2, which is predicted to have four possible reaction channels caused by different attacking orientations of HO2 radical to CH2O. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of the four reaction channels are calculated at the B3LYP/cc-pVTZ level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of some single-point multilevel energy calculations (HL). The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/ SCT) method. The calculated results show that, in the whole temperature range, the more favorable reaction channels are Channels 1 and 3. The total ICVT/SCT rate constants of the four channels at the HL//B3LYP/ cc-pVTZ level of theory are in good agreement with the available experiment data over the measured 10 temperature ranges, and the corresponding three-parameter expression is k(ICVT/SCT) = 3.13 x 10(-20) T-2.70 exp(-11.52/RT) cm(3) mole(-1) s(-1) in the temperature range of 250-3000 K. Additionally, the flexibility of the dihedral angle of H2O2 is also discussed to explain the different experimental values.