The invention provides a forming and die releasing device for a large length-diameter-ratio type composite product and a die releasing method. The die releasing device comprises a left half die, a right half die, a connecting pin bar, a screw rod, die releasing rings, a die releasing plate and a nut. When in the die releasing process, the nut is rotated to drive the die releasing plate and the corresponding die releasing ring to be in contact with the composite product; the quantity of the die releasing rings can be continuously increased to enable a die body to be released from the composite material product finally, so as to finish the die releasing. The device and the method are applicable to low-temperature, medium-temperature and high-temperature composite products; the product has the advantages of being high in processing accuracy, high in surface quality, and easy to be released from the die.