Extant research pays little attention to unorganized migrant workers' skill accumulation/upgrading from the perspective of the labor supply. This paper takes China as an example to explore the factors influencing the skill accumulation of rural-urban migrant workers (RUMWs), with the purpose of discovering how to sustain or reshape regional competitive advantages by improving RUMWs' skill accumulation. Structured questionnaire surveys were adopted for data collection in Suzhou City, Jiangsu Province and Taizhou City, Zhejiang Province located in the Yangtze River Delta in eastern China. In total, 700 questionnaires were issued and 491 effective questionnaires were recovered. It takes the perspective of individual laborers, with special regard to the effects of localization on the laborers' skill accumulation within the context of globalization. It adopts a broad viewpoint including intra-firm skill-biased strategy (as a response to intense competition), inter-firm relationships, and the accessibility of local non-firm organizations. The findings indicate that firms' skill preference, which impacts employees' skills and innovation ability and stimulates them to learn with initiative, have a significant influence on RUMWs' skill accumulation. In terms of collective efficiency based on the co-competitive relationship between local firms, the more intensive interactions are, the more opportunities RUMWs are afforded for skill accumulation. The accessibility of local institutions and favorable policies also benefit RUMWs' skill accumulation. In addition, the place itself, as a synthesized space of a firm's internal labor-management relations and inter-organizational relations, also exerts an influence on and causes regional differences in RUMWs' skill accumulation.