In this work, a novel GaN MIS-HEMT with highly suppressed short-channel effect is proposed and investigated. The device features a Source-connected clamp electrode near to the Drain-side Gate edge, which can clamp the voltage at the Drain-side Gate edge to less than 2V, subsequently suppressing the short-channel effect. Compared with the conventional short-channel GaN MIS-HEMT, the off-state leakage current of the proposed GaN MIS-HEMT decreases from 10-1 A/mm to 10-7 A/mm (with Gate voltage of 0V and Drain voltage of 10V), and the electron density under the Gate region is also greatly reduced (from 1018 cm-3 to 1011 cm-3), subsequently increasing the threshold voltage from the negative value to 2V. Moreover, the proposed GaN MIS-HEMT also delivers much lower reverse Gate-to-Drain capacitor due to the electrostatic shielding of the Source-connected clamp electrode. Both increase in the length and depth of the Source-connected Schottky clamp electrode can further reduce the off-state leakage current, the electron density under the Gate region, and the reverse Gate-to-Drain capacitor. The excellent characteristics of the proposed SC GaN MIS-HEMT show that the device is promising for future power applications.