To evaluate the click efficiency of different graphic designs of Virtual Reality (VR) interactive interface, a prediction method for the completion time of directional tasks in virtual scenarios was proposed based on probabilistic Fitts' law, and an arbitrary shape selection model of VR was constructed. First, according to the actual needs of VR interface design, the influence of shape on the completion time of directional tasks was added, a relation function between hit probability and task difficulty index was constructed, the target centroid was set as the center point of the function to be integrated, and the definition of probabilistic Fitts' model under the virtual scenario was completed. Then, the first experiment was designed to obtain the constant term value of the probability function in the improved probabilistic Fitts' model. On this basis, the second experiment was designed to calculate the constant term of the prediction function in the improved probabilistic Fitts' model, so as to construct the improved probabilistic Fitts' model. Finally, the model was verified and evaluated on the actual click tasks in a VR tobacco sorting system. Experimental results show that the model can predict the task completion time in the virtual scenario.