Research has revealed the positive impact of intelligent personal assistants (IPAs) on L2 learners' oral development and learning attitude. These studies, however, focused mostly on the in-class use of IPAs, with existing research on the out-of-class use being exploratory. To fill the gap of lacking empirical investigations on IPA-based autonomous second language learning (ASLL), this study recruited 34 college EFL learners to use Google Assistant (GA) in their respective homes for six weeks and examined their listening and speaking development afterwards. Each week, participants received an email about the new commands for them to explore, and half of them were additionally given sustained teacher guidance (i.e. weekly evaluation forms) on the usefulness of each command to better enhance their participation. Statistical analysis on the participants' learning gains demonstrated that participants receiving weekly evaluation forms made significant improvement in both listening and speaking skills, and they even outperformed those without weekly evaluation forms in their oral development. Nevertheless, there was no significant difference in the two groups' listening gains. These findings demonstrate that, with sustained teacher guidance, IPAs have great potential in L2 oral development in ASLL context, but more long-term studies on IPAs' impact for L2 listening should be conducted.