does any word exist for this?
I'd like to know if there is a word for this thing; let say, I am in a flexible working shift devided in daytime and nighttime. and when I am in a daytime shift, how should I call someone come after me as a night shift worker?

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I think you meant, "what should I call someone who comes after me as a night shift worker."

In that case, I don't think there is anything special you would call them. Coworker, of course. One thing you might call them is your, "relief," because they are there to relieve you of your duties for the day. That is not specific to night shift workers though. When you came back in the morning, you'd be their "relief".

I may have misunderstood your question though. Anyway, I hope this helped. Good luck!

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thanks a lot guys!

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"swing shift" is "back shift" in the UK.  Otherwise, all the terms I've seen so far are the same.

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I agree with Faye.  The 24 hour day is divided into 3 shifts: day, swing (evening to around midnight), and graveyard (midnight to 8 am).  Night shift is more common than swing because most companies don't have a 3rd shift of workers.

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Yes, we also sometimes refer to the shift in the middle of the night as the "Graveyard shift"!