The invention provides a rail length measurement device and a measurement method. The rail length measurement device comprises a down pressing wheel group and an encoder measurement group, wherein thedown pressing wheel group and the encoder measurement group are movably connected; the rail enters from one side of the encoder measurement group and slides out from one side of the down pressing wheel group; the down pressing wheel group comprises at least one down pressing wheel, and the height of the down pressing wheel is higher than the surface of the rail; the encoder measurement group comprises at least one encoder measurement wheel and an encoder; the height of the encoder measurement wheel is lower than the bottom surface of the rail; the encoder measurement wheel and the encoder arein coaxial connection; the rail slides above the encoder measurement wheel without contacting the encoder measurement wheel; the down pressing wheel is downwardly pressed to enable the encoder measurement wheel movably connected with the down pressing wheel to rise to press against the bottom surface of the rail; the encoder measurement wheel rotates along with sliding of the rail; and the encoder measures the sliding length of the rail through recording the rotation turns of the encoder measurement wheel. Due to the above device and the method, damage of the encoder measurement wheel due toimpact can be avoided, the service life of the encoder measurement wheel can be prolonged, and the rail measurement precision is improved.