Abstract: The technique on on-line weighing in the course of NaK alloy preparation was investigated. The design program on on-line weighing loop of NaK alloy preparation was established by theoritical analysis, and the devices were set up. The rectification equation was obtained by standard poise for on-line weighing. The correction equation and its weighing method are right which are proved by outline weighing for NaK after transfer. The method is suitable well for on-line weighing in the course of NaK alloy preparation.