I have install a python environment by virtualenv tool, let's say

/home/frank/env

and python is

/home/frank/env/bin/python3.

How to reuse the virtualenv by setting clearml agent?

So the agent is already caching the entire venv for you, nothing to worry about, just make sure you have this line in clearml:
https://github.com/allegroai/clearml-agent/blob/249b51a31bee97d63f41c6d5542e657962008b68/docs/clearml.conf#L131
No need to provide it an existing venv, if you are running in docker mode, you can provide
CLEARML_AGENT_SKIP_PIP_VENV_INSTALL=/home/frank/env/bin/python3