The invention discloses a process for continually preparing valeraldehyde, which reacts 1-butene raw material continually containing a little of trans/cis-2-butene, isobutene, butane with synthesis gas in the presence of the organic phosphinerhodium catalyst and triphenylphosphine. The process of the invention includes a reaction region, separation region and an activation region. Butene hydroformylation reaction liquid is evaporated, condensed and separated to two streams of raw product and circulating catalyst solution by film evaporation equipment, under shielding gas atmosphere of synthesis gas, and activated circulating catalyst solution enters reactor again. The present invention activates rhodium catalyst by catalyst activator, and controls or reduces acetal high boiling point byproducts which are generated by aldehyde polycondensation, enhance catalyst activity and regioselectivity of aldehyde. The film evaporation equipment used in the present invention is more effective than flash evaporator to separate catalyst solution and valeraldehyde producers in reaction liquor. Appropriate operational condition of film evaporation equipment can reduce or not generate aldehyde condensation, protive rhodium catalyst, enhance catalyst activity and prolong usage time of catalyst.