Adding mesophyll conductance to the land surface model can effectively improve the accuracy of CO₂ fertilization effect in the model. However, the simulation of mesophyll conductance is affected by the maximum mesophyll conductance parameter and the maximum mesophyll conductance is a key parameter to improve the land surface model for mesophyll conductance and CO₂ fertilization effect simulation. In this study, we added mesophyll conductance to the EALCO (Ecological Assimilation of Land and Climate Observations) model. By changing the maximum mesophyll conductance values artificially, we analyzed the responses of the model outputs to the variable maximum mesophyll conductance values, explored the sensitivity of the maximum mesophyll conductance parameter in the model and compared the results with some existing researches or observation data. Based on the comparisons, we explored the way to optimize the maximum mesophyll conductance parameter of land surface model. The simulation experiment was driven by the oberservation of a typically temperate deciduous broad-leaved forest ecological station in the Harvard Forest Environmental Monitoring site (US-Ha1 site). The results showed that the gross primary production (GPP) simulation accuracy increased with the increase of the maximum mesophyll conductance value. When the maximum mesophyll conductance was greater than 1.0 mol m^-2 s^-1, the improvement of the simulation accuracy was limited, while the GPP simulation accuracy was sensitive when the maximum mesophyll conductance value was less than 1.0 mol m^-2 s^-1. The results also confirmed that there was an obvious linear relationship between mesophyll conductance and stomatal conductance. The changes in the value of the maximum mesophyll conductance affected the slope of linear relationship obviously. When the value of the maximum mesophyll conductance increased from 0.5 mol m^-2 s^-1 to 1.2 mol m^-2 s^-1, the ratio of stomatal conductance to mesophyll conductance decreased from 0.75 to 0.36. The result suggests that by clarifying the ratio of mesophyll conductance to stomatal conductance of plant functional types, the reasonable value range of the maximum mesophyll conductance in land surface models can be determined indirectly. Our results also confirmed that adding mesophyll conductance to the EALCO model could improve the simulation accuracy of CO₂ fertilization effect, and the maximum mesophyll conductance could affect the simulation of CO₂ fertilization effect. When the maximum mesophyll conductance value was greater than 0.57 mol m^-2 s^-1, the growth rate of GPP with the increase of atmospheric CO₂ concentration showed a downward trend with the maximum mesophyll conductance value increasing. We found that the sensitivity of mesophyll conductance simulation to the maximum mesophyll conductance changes was different in different growing seasons. The maximum mesophyll conductance in the peak growth season (July and August) has the greatest impact on the simulation of mesophyll conductance, less impact in the sub-peak growth season (May, June, and September), and little influence in other months.