Based on the power coupling theory, the characteristics of optical signal power and inter-core crosstalk (ICXT) are studied in weakly coupled multicore fiber (MCF) transmission systems with multiple core interference. The corresponding analytical expressions of the optical signal power and ICXT are also derived. Simulation results show that, after long distance longitudinal transmission, the optical signal power in each core will reach a state of dynamic balance, and a calculation formula for the dynamic balance normalized power is proposed. Moreover, in the MCF transmission systems with multiple core interference, the ICXT contributions induced by different incident cores of the MCF are uncorrelated. The ICXT distribution in the coupled fiber core can be regarded as the accumulation of the ICXT of multiple dual-core single-input cores. Based on the cumulative characteristics of the ICXT, a number of generalized crosstalk estimation mathematical models are obtained, which improve the analysis theory of ICXT in the case of multiple core interference and provide a good theoretical analysis tool for this situation.