In construction scheduling, constraints among activities are vital as they govern the schedule solution. Understanding their criticality is essential for better schedule management. This paper presents a systematic method to classify and identify the criticality of schedule constraints for the schedule management from the constraint perspective. In terms of criticality, schedule constraints can be grouped into four types: project-critical, activity-critical, sequence-critical and non-critical. Project-critical constraints are those which govern start/finish time of critical activities and the project end time. Activity-critical constraints define the start/finish time of non-critical activities, and sequence-critical constraints are those whose existence affect the start/finish time of some activities or the project end time. Constraints belonging to any of these groups are vital to a schedule as they cannot be removed from the constraint collection. Noncritical constraints, on the other hand, do not govern either start/finish time of any activity or the project end time. Accordingly, non-critical constraints are redundant and can be removed from the constraint collection without causing any change to the schedule solution. The method proposed was applied to a illustrative case example based on the construction of the main entrance of a nursing house for further interpretation. The proposed classification scheme could shed light on a more in-depth understanding of the nature of criticality and the role of constraints in a schedule, and thus better schedule management may be achieved.