We are planning to sleep nearby Shantang Street, Suzhou. Can we walk around at night? is it safe?

Yes, you can have a walk along the street. It is quite safe. The locals are quite friendly and warm-hearted. Thus, you don't need to worry about the safety problem. However, you are not advised to walk outside at the midnight.
In addition, you can easily find many stores and stalls there along the street. Have fun! :)

what time lamps get turn off at night in Shantang st?

As I know, there may be some lights on all night. But most stores close at around 21:00, while the bars and cafes will operate until around 02:00am.