The paper aims to investigate ammonia/hydrogen premixed combustion in order to assist a high-efficient and low emission combustor design of a novel Linear Engine Generator. Characteristics of ammonia/hydrogen premixed combustion are revealed using a detailed chemical kinetics mechanism. An ammonia combustion mechanism is identified among many mechanisms and validated with published experimental data. In order to investigate the combustion characteristics under working conditions of the Linear Engine Generator with both internal and external combustor, a parametric analysis is carried out to study the effects of equivalence ratio (0.8-1.6), hydrogen blending ratio (0.0-0.6), initial temperature (300-700K) and initial pressure (1-20bar) on premixed laminar flame speed, ignition delay and concentrations of main pollutants. It is shown that an equivalence ratio of around 1.1-1.2 is beneficial to ammonia flame stability and cleaner emissions. Ignition delay is reduced and flame speed is enhanced with the increase of hydrogen blending ratio and initial temperature. However, excessive hydrogen blending (over 0.4) and ultra-high temperature result in the unburnt H2 and NH3 respectively, which would decrease the efficiency of combustion and the Linear Engine-Generator. It is also found that although high-pressure environment reduces the flame speed, ignition delay is shortened and NO emission is reduced considerably.