Based on the coupling characteristics of slot waveguides and the thermo-optical modulation, an ultra-compact microring resonator on silicon-on-insulator with the size of 15 μm×20 μm is proposed, whose bandwidth can be tuned by adjusting the heater power. The tunability characteristic is simulated through finite difference time domain method, the result demonstrates when the wavelength of incident light is 1 550 nm, its adjustable bandwidth range is 2.5 nm at 1 900 nm wavelength for TE mode and 3.1 nm at 1 543 nm wavelength for TM mode by applying the heater power from 0 mW to 0.742 mW. The proposed device with small size, low driving power and wide tunable range of bandwidth is very promising for dynamic integrated optical signal processing such as reconfigurable filtering and routing.