In the double-cone ignition scheme of inertial confinement fusion, the head-on collision of two compressed fuel jets from the cone-tips forms an isochoric plasma, which is then heated suddenly by an MeV relativistic electron beam produced by ultra-intense picosecond laser pulses. This fast-heating process was studied experimentally at the Shenguang II upgrade laser facility. By observing temporal resolved x-ray emission and spatial resolved x-ray spectrum, the colliding process and heating process are carefully studied. The colliding plasma was imaged to have dimensions of ~86 μm in the implosion direction and ~120 μm in the heating direction. By comparing the simulated plasma x-ray spectrum with experimental data, the electron temperature of the heated plasma was found to rapidly increase to 600±50 eV, almost doubled temperature achieved before the heating laser incidence.