The invention discloses a composite potassium carbonate fertilizer and a preparation process thereof. The composite potassium carbonate fertilizer is prepared from, by weight, 25-30 parts of urea, 35-45 parts of monoammonium phosphate, 20-30 parts of potassium carbonate, 2-3 parts of ammonium bicarbonate and 2.5-3.5 parts of calcium sulfate. The preparation process comprises the steps that smashing is performed, wherein the urea, the monoammonium phosphate, the potassium carbonate, the ammonium bicarbonate and the calcium sulfate are smashed by a smashing machine according to the weight parts; stirring is performed, wherein the obtained mixed smashed materials are fed into a stirring tank through a conveying belt for stirring, and the materials are conveyed into a granulator after being evenly stirred; steam is introduced into the granulator, the viscidity of the monoammonium phosphate in the materials is increased through the effect of the steam, and mixed granulation is performed; the obtained granulated mixture is dried, screened and cooled, and finished product screening is performed to obtain finished product granules; the outer layers of the obtained finished product granules are coated with an anti-blocking substance through a coating machine, and then package is performed. A product forming period can be shortened by replacing the potassium carbonate with traditional potassium sulfate, and the yield of the compound fertilizer can be increased.