Abstract: In previous studies, the uncertainty of net load is often characterized from the time point. For example, the uncertainty at the time point can be described by probability distribution function or interval, and there are relatively few researches on the uncertainty of the net load fluctuation process between adjacent periods. Aiming at the uncertainty of the net load fluctuation process between adjacent periods, this paper explores and characterizes the extreme net load fluctuation trajectories from fluctuation quantity and fluctuation speed, and proposes a unit commitment model that considers the uncertainty of the net load fluctuation between adjacent periods. The net load fluctuations between adjacent periods are described by four kinds of extreme net load fluctuation trajectories to ensure a robust response to net load fluctuation between adjacent periods, avoiding overly aggressive results. Meanwhile, the power flow constraints under the extreme fluctuation trajectories are also included in the model to ensure the deliverability of the reserve required for net load fluctuations. Case studies show that the model proposed in this paper can more reasonably allocate the unit climbing capacity required by net load fluctuation, and can ensure the safe and economic operation of the power system under extreme fluctuation.