on second thoughts 转念间，经过重新考虑后(突然改变主意)，再次考虑后

【例句】
On second thoughts, however, he said that he was so ill-tempered that most of his students avoided making him mad.
经过再三考虑,他说他脾气很坏,他的大多数学生都避免惹怒他。

On second thoughts, I decided to study medicine.
经再三思考，我决定学医。