>> I plan to walk around St.Moritz lake in the morning (9 am), but I confuse about the distance around the lake, some said 1.8 km., some said 4-5 km.
It's an easy 4.4 km walk taking around an hour. More details here:
>> After finished St.Moritz lake I’ll catch the train to Winterthur, time to arrive before 5 pm., along the way to Winterthur is it possible to do one or two city tour, please recommend where should I visit and at what time should I depart from St.Moritz just to fill the trip.
The trip from St. Moritz to Winterthur will take close to 4 hours. The two fastest routes are:
St. Moritz - Chur - Zürich - Winterthur (3 hours 55)
St. Moritz - Landquart - Zürich - Winterthur (3 hours 41 min)
Given your time constraints, the length of the journey, the frequency of the trains, and the towns you go through, I can't really think of anywhere suitable to explore on your way back to Winterthur. Perhaps someone else can come up with an idea.
A way to make your trip more interesting though (without stopping off anywhere) could be to incorporate a trip over the Flüela Pass by PostBus. I have attached an itinerary that could work, and a map showing the route. I assume you will be doing this trip in mid-September (as this route is not possible all year round).
The route is:
St. Moritz to Susch by train
Susch to Davos Dorf by PostBus over the Flüela Pass
Davos Dorf to Winterthur by train (change at Landquart and Zürich)
If you are interested, you can read more about this suggested route (and see some photos) in the following posts:
Postbus over the Flüela Pass
St. Moritz to Zürich via Landquart
My post of Feb 28, 2017 - 4:36 AM (and the following one) on this page:
>> Just one more question is Swiss Travel Pass be foldable?
Yes, you can fold it, but not through the matrix barcode, which the conductor needs to read with a scanner.