The closing speed is an important parameter of mechanical characteristics of vacuum circuit breakerand it is difficult to monitor it on line. In view of this problem

the idea of closing speed calculation based on its closing stroke curve is proposed. First

the off-line measurement test of closing speed of 12 kV vacuum circuit breaker is designedand thedata of closing stroke curve is obtained. Then

two kinds of calculation method of closing speed relying only on the stroke curve is proposed. The first method adopts the least square method to perform piecewise fitting of the closing stroke curve

which can calculate the closing speed without obtaining the information of the instant point of contact closing. The second method fits the stroke curve in accordance with the motion of contact in the closing process and by the use of B-spline function

finds out the instant point of contact closing and calculates the closing speed. In the two methods

the accuracy obtained by the B-spline fitting is higher

while the consumed resource by the least square fitting calculation is less. The proposed methods are simple

have high accuracy and can select in accordance with actual condition of projects

which have provided new idea for studying the on-line monitoring of vacuum circuit breaker.