Ammonia (NH3) can be used as carbon-free alternative fuel for modern energy and transportation systems. Co-firing NH3 with syngas can overcome the high ignition energy and low burning velocities of pure NH3 flames on the one hand, while regarding the characteristics of syngas on the other hand, this strategy may have low-emission potential in real application, and a corresponding research can be helpful for validating or developing NH3 co-firing mechanisms with more complex fuels. The present study experimentally investigated laminar burning velocities of NH3/syngas/air flames at atmospheric pressure and 298 K using the heat flux method. Two types of syngas components were used, i.e., SYN_A: 5 vol% H2 + 95 vol% CO and SYN_B: 50 vol% H2 + 50 vol% CO, and the measured conditions cover wide ranges of mixing ratios and equivalence ratios. Several literature kinetic mechanisms were tested and a new mechanism was proposed. Results calculated by the present mechanism agree well with experimental data of the burning velocities and the ignition delay times of NH3, NH3/H2, NH3/CO, and NH3/syngas flames at various mixing ratios, equivalence ratios, and pressures. The present mechanism also reproduces the trend of NOx emission characteristic in literature. Detailed kinetic analyses using the present mechanism were carried out, showing the NH3 oxidation processes in NH3/syngas/air flames and the most rate-limiting reactions for predicting the laminar burning velocities. Important reactions with different rate parameters from different sources were labeled, which could be helpful for future organization or optimization of NH3 kinetic mechanisms.