Hi, I am flying from Nepal to Kunming then Chengu in Feb. Which will be shorter route to Jiuzhaigou

I'm afraid the railway from Chengdu to JZG is still under construction.
For shorter route, you can take an airplane flying from Chengdu to JZG Airport, where you can take a shuttle bus to the scenic area within about 1.5h.

Or upon arrival at Chengdu, you can go to the local Xinnanmen Bus Station to catch a long-distance bus to the scenic area within 8-9h.

As for the weather in Feb. it will be cold, with the highest temperature of about 13C and the lowest temperature of -10C. The scenic area may be a little colder. So you'd better take along winter clothes, such as down jacket, sweater and thermal Long John. It may also snow a little. So do make a good preparation.