If i stay in lanzhou, can i visit zhangye on a day trip?

A round trip in one day can be tight. You can take the earliest bullet train to ZY first. It takes around 5h. Then you can hire a taxi or private car to the attraction within an hour. The round trip may cost CNY150-200. 3-4 hours can be enough for touring around. The park closes around 19:00. So you will have enough time. Then stay a night in ZY City and go back to Lanzhou in the next day.