Science fair is one of the most common open inquiry activities which can facilitate learners to construct their science knowledge and develop science literacy in school. However, there are a great deal of difficulties and challenges in Taiwan's science fair. For example, novice teachers may neither effectively guide learners to conduct science fair inquiry activities nor facilitate learners to construct related knowledge. To scaffold teachers' instruction and students' learning in science fair inquiry effectively, the "Online Science Fair Inquiry System based on scaffolding design" (OSFIS v 2.0) adapted from (OSFIS v 1.0) was developed in this study. After the development of the system, this study also conducted a series of system evaluations on it. To this end, questionnaire survey was conducted. The participants of the system evaluation in this study were 61 elementary school teachers. The participants expressed highly satisfactory perceived usefulness on teachers' and students' scaffolding design. Also, they had high intention to use the OSFIS v 2.0. Moreover, this study reveals that the system may facilitate both teachers and students to understand the process of science fair inquiry, solve the limitation of activities times, record the portfolio during inquiry activities, and complement teachers' professional knowledge. Finally, some suggestions and implications for teachers to conduct open inquiry activities, system design, and future work are also proposed.