The reason this error occurs is because Python packages in /opt are added to PYTHONPATH environment variable BUT for ones in /usr, path does not exist in PYTHONPATH variable. The python-rospkg gets installed in /usr by default. So you just need following line in .bashrc to fix this:
export PYTHONPATH=$PYTHONPATH:/usr/lib/python2.7/dist-packages
ok,it can work for me