The influence of fluorine or/and aAnne substituents to the mechainsm of cycloaddition reaction of ethylene and formaldehyde has been studied by using senilemPirical molecular orbital theory UAM1 presented by M.J.S. Dewar et al in 1985, and performed with GAUSSIAN 86 program. Three chendcal reactions (C_2H_4+COF_2, H_2NHC=CHNH_2+H_2CO and H_2NHC=CHNH_2 +COF_2) have been studied in detail. Thevarious critical points of all the reactions studied have been fully optimized with Berny gradients and transtiton structures have been confirmed by frequency analysis. From the results we concluded that: 1) All the reaction pathways studied in this work are found to proceed via a two-step approach characterized by diradical gauche intermediates and the rate determining step is the first step for all the three reactions. 2) The fluoride or/and amine substituents can all reduce the potential energy barrier of the cycloadditon reaction of ethylene and formaldehyde, and amine is more favourable than fluoride.