The invention provides a method for obtaining constant elongation in states of acceleration and deceleration. The method comprises the following steps of respectively acquiring the speed of
strip steel of an entrance and the speed of
strip steel of an exit; setting a length-variable calculating period; calculating an actual value of the elongation; comparing the actual value of the elongation to a setting value of the elongation to obtain an elongation difference value; using a product of the elongation difference value and an elongation adaptive adjustment factor as an inputting value of a PI (proportional integral) controller; outputting additional controlled quantity of a rolling force control ring and additional controlled quantity of a
tension control ring; and superposing the additional controlled quantity of the rolling force control ring, the additional controlled quantity of the
tension control ring and a rolling force adaptive adjustment factor to obtain final rolling force adjustment amount. Elongation length calculating periods are set at different speed sections, so that the problem of slipping of
strip steel in processes of acceleration and deceleration can be solved. By correlation operation of three-times extraction of a square root of actual speed, elongation fluctuation adjustment in the processes of acceleration and deceleration is accelerated in a feedforward
control manner; and adaptive factors are set according to dynamic correction of errors of an actual rolling force and an elongation expected rolling force and relation of thickness, width and elongation, and the control precision and the stability of the elongation in the processes of acceleration and deceleration are improved.