If you describe something as unnatural, you mean that it is strange and often frightening, because it is different from what you normally expect.
The aircraft rose with unnatural speed on takeoff.
飞机以反常的速度起飞升空。
Behaviour that is unnatural seems artificial and not normal or genuine.
She gave him a bright, determined smile which seemed unnatural.
她开朗、坚定地对他笑了笑,但看起来不太自然。