A criterion for predicting the flame spread time of stick-shaped Mg alloy samples was proposed based on the law of energy conservation. It is indicated that the flame spread time is mainly affected by the material and sample dimension. The initial temperature and ignition temperature of the control volume and the S/l_c ratio (the cross-sectional area/cross-sectional circumference) of the sample are decisive for the flame spread time. The flame spread time is proportional to the ΔH (the energy required to heat a unit mass of sample from initial temperature to ignition temperature) and the S/l_c ratio, which is confirmed in three Mg alloys of Mg-8.5Al-0.5Zn-0.2Mn, Mg-2.7Nd- 0.4Zn-0.6Zr and Mg-4.0Y-3.3Nd-0.5Zr. For samples with the same dimension, the Mg-8.5Al-0.5Zn-0.2Mn alloy sample possesses the largest ΔH and the longest flame spread time. For samples with constant length and cross-sectional area, the sample with circular cross-section has the largest S/l_c ratio and the best flame spread resistance in the same alloy.