The invention provides a method for synthesizing propyl aldehyde by ethylene hydroformylation. The invention belongs to preparation of a saturated compound which contains a -CHO group which is connected to an acyclic carbon atom. In the method, the ethylene, carbon monoxide and hydrogen are taken as raw materials. The method comprises the following steps: adopting a rhodium phosphine complex catalyst system and ethylene hydroformylating to synthesize propyl aldehyde. The invention is characterized in that the system comprises a synthesis operating unit, a separation recovery operating unit and a rectification operating unit; and in the method, the complete separation of the synthesized propyl aldehyde product from the catalyst is realized; and the method has the advantages of low catalystconcentration, low consumption, low reaction temperature, long service life of the catalyst, high raw material conversion, less impurities in the propyl aldehyde product and high propyl aldehyde content. The synthesized propyl aldehyde product is completely separated from the catalyst, thus avoiding catalyst loss, reducing catalyst consumption, avoiding decomposition of the catalyst and prolonging the service life of the catalyst. A double-synthesis reactor is adopted, raw materials are recycled, feed gas conversion rate counted by ethylene is more than or equal to 95%, and propyl aldehyde content is more than or equal to 99.9%.