To address the complex harmonic coupling characteristics exhibited by a large number of power electronic converters in large-scale photovoltaic power plants under different operating conditions, a harmonic state-space-based modeling and analysis method is proposed. First, considering the electrical connections among multiple generation units in large PV power plants, a modular and scalable reduced-order harmonic state-space model of the grid-connected PV system is established. The proposed model is then used to analyze the interaction mechanisms of harmonic coupling in the grid-connected system. By accounting for variations in grid strength during the practical planning and operation of PV power plants, the harmonic coupling characteristics of grid-connected PV power plants under different grid strengths are investigated. The results indicate that the interaction strength of harmonic coupling mainly depends on the degree to which harmonic sources affect the voltage at the point-of-common-coupling. Finally, a time-domain simulation model of a large-scale photovoltaic grid-connected system is developed, and the simulation results verify the accuracy of the harmonic state-space model and its applicability for harmonic coupling analysis.