First possession is a common heuristic people use to solve property conflicts. Previous studies examined whether young children judged ownership based on the first possession heuristic and its stability when conflicting with other cues such as labor, but few focused on the effects in the discovery context. In this study, we used two discovery stories which indicate the discovered object was not owned by anyone beforehand and investigated ownership reasoning with the first possession heuristic in Chinese 3- to 6-year-old preschoolers. By pitting the first possession cue against the labor cue, we investigated the stability of the first possession heuristic in young children's ownership reasoning. The results showed that in the condition where there was only the first possession cue, both the younger and older groups used the first possession heuristic to reason about ownership. However, in the labor condition, 5- and 6-year-olds ceased to support the first possessor and turned to assign objects to the laborer, whereas 3-year-old children still insisted on the first possession heuristic (Study 1 and Study 2). Children across four age groups did not assign ownership to the person who just played with the object but did not modify it (Study 2). The results demonstrate that Chinese preschoolers understand the role of first possession in ownership assignment at an early age in the discovery context but the elderly preschoolers do not rely on the first possession cue when there are conflicting cues such as labor.