The invention relates to a height-measuring and weighing device for a forklift and a weighing method. The device comprises a sensing unit, an input unit, a central processing unit (CPU), a storage unit, an output unit and an execution unit, wherein the sensing unit is provided with two photoelectric encoders arranged on chain wheels of a middle cylinder and an edge cylinder respectively; the CPU is used for obtaining a Lagrange interpolation polynomial between output data of weighing and the weight of loaded goods, obtaining the weight of the goods according to the Lagrange interpolation polynomial and an input pressure signal, obtaining an increment according to counters of the two photoelectric encoders of the sensing unit, converting the increment into a corresponding height value, correcting a previous height value according to positive and negative rotation information of the chain wheel and updating the height value; and the storage unit is used for storing the Lagrange interpolation polynomial calculated by the CPU. The device has the advantages of automatically ascending or descending a fork of the forklift by setting a height and automatically alarming when the goods are overweight, along with high weighing accuracy.