Objective To investigate the relationship between different brain developmental stages and changes of cognitive function in rats. Methods 1?month, 2?month and 8?month?old rats were selected to imitate the juvenile, adolescent and adulthood, respectively, and their behavioral functions were compared. The reward operant conditioning and Morris water maze task were used to investigate the differences in exploration interest, executive and recognition ability, spatial learning and memory of the rats at different ages. Results In the reward operant conditioning and Morris water maze task, there was no significant difference in the cognitive function between 1?month and 2?month?old rats. In the reward conditioning phase, the nose pokes numbers of 8?month?old rats were significantly decreased compared with the 1?month?old rats ( P < 0.01). There was no significant difference in nose pokes accuracy. During the operant conditioning phase, the lever press numbers and accuracy of 8?month?old rats were significantly decreased ( P < 0.05 or P < 0.01) and the press latency was longer ( P < 0.05). At the phase of visual identification, the press and reward numbers, and the visual identification index were significantly decreased ( P < 0.05 or P < 0.01). In the Morris water maze test, compared with the 1?month?old rats, the total swimming distance and escape latency of the 8?month?old rats were significantly increased ( P < 0.05), as well as average swimming speed ( P < 0.05 or P < 0.01) in spatial learning phage. In spatial memory phage, the swimming distance and time spent in the target quadrant were obviously decreased ( P < 0.01). Conclusions The cognitive functions of rats at different brain developmental stages are different. The juvenile and adolescent rats have similar cognitive functions, but 8?month?old adult rats appear decline in the exploration interest, executive and recognition ability, and spatial learning and memory function.