Thank you for your reply Ted, which is very informative.
As for the word template, it only need short input from the form, such as a length of the product '56mm', or name of some content 'moisture'. The word template is big, with all the descriptions, but only these basic words are needed through the form.
The tree sturcture is similar to below chart

on each node, user chooses on path down to another node, until the final leaf node. In other words, the infopath should ask the user a question, while when the user moves down along the path, the next question maybe different from time to time, because user picks different answers (paths) each time.
And this is the meaning of 'dynamic' i think. Can I say this is a kind of decision tree, but we have already had the tree structure, and only have to follow the existing rules.
All of these are not very hard, but i am wondering is there are formal way to design it... since it would take a long time to do this in a pure programming environment...