The invention relates to a method for eliminating the side effect of dehydrogenation from carbon dioxide to urea device. It is characterized in that it employs oxygen to replace normal air to be used as oxidant for dehydrogenation, the maximum oxygen amount is oxygen consumption amount for dehydrogenation, the addition point is any position including carbon dioxide compressor import, each section, export and place before dehydrogenation reactor. On one hand, the invention can solve the shortcomings of reduced production ability of urea device caused by application of normal air, on the other hand, it can increase the system stability, operational elasticity and reduce device energy consumption.