I want to know if there is taxi from hangzhou airport to the town at midnight?

Yes, the taxi is still available at the airport at midnight. You can easily take that out of Door 10, Arrival Floor of Domestic Terminal. The taxi fare shall be around CNY100 to the mail railway station.

If you are going to travel alone, it is best to travel by shuttle bus, which will still in service upin your landing and it will directly take you to the train station, costing CNY20.