The invention relates to a total-dissociation DGT combined film for determination of soil available phosphorus and a preparation method thereof. The total-dissociation DGT combined film comprises the material composition: available phosphorus combined material nano Mg-Al-Fe-LDHs, a combined material dispersing agent, an acrylamide gel and the like. The above materials are uniformly mixed through ultrasonic treatment, and the mixture is pumped into a high-silica glass planar groove provided with a certain thickness of gaskets at periphery, after a mouth is sealed, a reaction is carried out for at least 1 h at the temperature of 42 to 46 DEG C, and the DGT combined film is prepared. The combined film material can highly efficiently and rapidly enrich available phosphorus, and can be completely dissociated within 30 min after 8 ml-10 ml of concentrated nitric acid is added; the combined film overcomes the disadvantages that a traditional DGT combined film is slow and incomplete to dissociate, difficult to prepare in laboratory, poor in generality, poor in phosphorus adsorption ability, high in price and the like.