The invention discloses a preparation method of a 5, 6-dihydrogen-pyran and the derivatives of the 5, 6-dihydrogen-pyran. In the present methods, the self-polymerization of conjugate dienes is easily happened in a synthesis reaction, and as a result, the yield rate of products is low. In the invention, under the catalysis of lewis acid, the conjugate dienes and anhydrous formaldehyde synthesize the 5, 6-dihydrogen-pyran and the derivatives by the Diels-Alder reaction; after being depolymerized by paraformaldehyde, trioxymethylene or polyoxymethylene in another reaction kettle, the anhydrous formaldehyde in the form of gas enters into the reaction kettle of synthesis reaction which is carried out under the temperature between 30 DEG C and 100 DEG C. Because the invention separates the depolymerization from the synthesis, the synthesis reaction does not have to be carried out under a high temperature, the self-polymerization of conjugate dienes is avoided, and therefore the yield rate of reaction is increased.