The invention relates to a nuclear turbine low-pressure rotor expansion quantity laser measurement method which includes the steps: arranging a first laser measurement standard ball at the initial endof a low-pressure rotor, and arranging a second laser measurement standard ball at the tag end of the low-pressure rotor; measuring coordinates of the first laser measurement standard ball and the second laser measurement standard ball by a laser tracker, and measuring basal heat deformation delta d of the low-pressure rotor; measuring heat expansion quantity delta L relative to the basal heat deformation of the low-pressure rotor by an absolute expansion probe; calculating an absolute expansion value of the low-pressure rotor according to the formula S=delta L+delta d. A connecting line of the first laser measurement standard ball and the second laser measurement standard ball is parallel to a turbine shaft system. The steps are simple, the laser measurement method is easy to implement,the expansion quantity of the low-pressure rotor can be accurately measured, intervention of operating personnel in a unit is decreased, power reduction and switch-off stop risks of the unit are avoided, and reliability of equipment and the power generation benefit of the unit are improved.