The present invention provides an operational-task-oriented system and method for dynamically adjusting operational environment applicable to a computer cluster. Each operational node of the computer cluster has two or more operational systems installed. After receiving the operational task, the control node estimates the time required for completing different tasks requiring different operational systems by appropriate operational nodes and compares the estimated finish time and the assigned finish time for judging how to adjust the operating system running in the operational nodes. Thereby, the operational task can be completed in the assigned finish time. Another method is to use the control node to analyze the proportions of the tasks requiring different operational systems in an operational task and hence adjusts the operational system running in an operational node according to the proportion of requirement. Thereby, the operational task can be completed in the shortest time.