In tokamak plasma merging experiments, high-power ion heating is achieved only when the current sheet thickness is compressed to ion gyroradius ρi. The magnetic reconnection converts 40%–50% of the reconnecting magnetic field energy (Brec2/2μ0) into the ion heating energy (Wi) with the scaling law of Wi proportional to Brec2. If the compressed current sheet thickness is larger than ρi the magnetic reconnection converts only 5%–10% of Brec2/2μ0 into Wi. The high-power ion heating can be understood from the experimental finding that the poloidal electric field due to electrostatic quadrupolar potential in the downstream region increases linearly with both Brec and the guide field Bt.