A numerical simulation model of the temperature field in the laser phase-transformation hardening of the highly-enhanced diesel engine valve seats is established and the influences of different process parameters on this temperature field are investigated in the laser phase-transformation hardening process. The results show that, with the increase of the laser power, the peak temperature of the phase-transformation hardening of the valve seats increases, the temperature field presents a cometary shape with a tail, and the depth and the width of the hardened layer increase synchronously. In contrast, with the increase of the scanning speed, the peak temperature of the phase-transformation hardening of the valve seats decreases, the cometary tail of the temperature field gets smaller, and the depth and the width of the hardened layer decrease synchronously. Moreover, with the increase of the laser spot radius, the peak temperature of the phase-transformation hardening of the valve seats decreases and its corresponding position shifts, the cometary tail of the temperature field gets larger, the depth of the hardened layer decreases.