In this work, we performed periodic Density Functional Theory calculations to explore reaction pathways of ethanol to 1,3-butadiene on ZrO2 surface. The overall energy barrier and reaction energy were explored for aldol condensation mechanism and Prins condensation mechanism. In particular, we identified that aldol condensation mechanism is dominant compared with Prins condensation mechanism on ZrO2 surface. We explained the better catalytic performance of two-step process from ethanol to 1,3-butadiene than one-step process due to higher reactivity of acetaldehyde added into reactants. It is verified that ethylene is by-product instead of intermediate product. In Meerwein-Ponndor-Verley reaction, crotonaldehyde can promote the formation of acetaldehyde and suppress the formation of ethylene from ethanol. Finally, we revealed that proton transfer step from carbon chain to catalyst surface is the rate-limiting step during acetaldehyde production from ethanol on ZrO2 catalyst. Therefore, we proposed that doping a second catalytic dehydrogenation promoter into ZrO2 based catalyst would improve the performance of catalyst.