In order to measure the diameter of the optical microfibers (OMs), the liquid immersion intensity detection method was used. Firstly, the structure of OM-liquid cylindrical waveguide was analyzed theoretically, and the relationship between the additional loss and the shape parameters of the whole OM was obtained. Then, an experimental system was set up to test two groups of OMs with different diameters. The experimental results show that when the diameter was 2.7μm, the minimum resolution of diameter measurement is about 3nm and the diameter offsets of four OMs are 40nm, 30nm, 80nm and 20nm, respectively. When the diameter is 4μm, the minimum resolution of diameter measurement is about 12nm and the diameter offsets of four OMs are 60nm, 90nm, 10nm and 30nm, respectively. At the same time, the results showe well diameter consistency and verified the feasibility of liquid immersion intensity detection method. This research provided technical support for the further development of OMs.