I used r and igraph 1.3.0 verson to analysis my data, the problem is show below, but I don‘t know why it is happened, can someone help to solve this problem? thanks.

Please always show a complete minimal example, otherwise it is very hard to tell what happened.

A possible cause is that you requested a smaller number of clusters than the number of connected components.

I now see that the R interface does not provide an explicit way to request a certain number of clusters with walktrap. Please show a complete minimal example so that we can reproduce the problem.

FYI, I found a small reproducer: Walktrap community detection fails on some disconnected weighted graphs · Issue #2042 · igraph/igraph · GitHub

Got it, thanks, hope it can solve my problem.