Children’s outdoor free play, which is characterized by intensive physical engagement and diverse social interactions, plays a unique role in early childhood development and education. However, existing scales cannot comprehensively measure children’s performance in outdoor free play. The research purpose of this study was to develop and validate an Outdoor Free Play Scale for Children-Preschool Version (OFPS-P) with good reliability and validity, in order to provide a practical tool for teachers to understand the level of children’s outdoor free play. Based on the review of existing scales of children’s play and the uniqueness of children’s outdoor free play, we developed a scale with 12 items and validated the scale with two samples of preschool children with exploratory (nsample1 = 140) and confirmatory (nsample2 = 241) factor analyses. Four factors were identified in this scale: physical fitness, approaches to learning, social interaction, and imagination. The results indicated good reliability and validity of OFPS-P, which can be used to evaluate preschool children’s performance on outdoor free play and to support teachers’ effective support in outdoor play activities in kindergartens.