The invention provides an optical measuring device. According to the optical measuring device, an optical detection module is arranged above a substrate; the optical detection module is provided witha parameter detection group; and the parameter detection group includes a mark position measurement module, a line width measurement module and a photoresist thickness measurement module. As for the flat panel field and patterns smaller than 2 microns, since the photoresist thickness measurement module and the line width measurement module are combined in the same parameter detection group, when correlation between line width and photoresist thickness is analyzed, the photoresist thickness measurement module and the line width measurement module are corresponding to the same position simultaneously since the devices are both installed on the same optical detection module, and therefore, analysis can be benefitted. With the measuring method provided by the invention adopted, an optimal environment for analyzing the correlation between the line width and the photoresist thickness can be found out, blind analysis can be avoided, and efficiency can be improved.