Duō nián de jūnlǚ shēngyá gǎibiàn le tā de xíxìng.
Years of military life changed his character and habits.
Tips
usage
习性 refers to traits that have formed over time through habit or environment - a blend of 习惯 (habit) and 性格 (character). It is often used to describe animal behavior or deeply ingrained human tendencies.