This invention provides an injection-molded plastic optical lens comprising: an optical effective region and a peripheral region that are concentrically formed. An outer edge of the peripheral region comprises an indentation section, and an accommodation space surrounded by the indentation section of the outer edge of the peripheral region and an extending line extending from the non-indented section of the outer edge of the peripheral region. The accommodation space confines a flash generated during the injection-molding process of the plastic optical lens so that the assembling of the present plastic optical lens will not be affected. The present invention employs a mold having an air duct connected to an indentation configuration of the mold. The indentation section of the plastic optical lens is formed through a mold replication of the indentation configuration.