Based on linear wave theory, the hydrodynamic problem of the array of truncated cylinders with relative motion is studied, and the calculation formulas of hydrodynamic force/moment, added mass, and added damping coefficients are given. Firstly, the effect of the evanescent mode on the hydrodynamic coefficients are studied by the truncated two-cylinders as an example. And based on this, an approximation formula for the heave added mass of the truncated two-cylinders is given. After that, a fast formula for calculating the added mass coefficient of an oscillating cylinder of the rectangularly arranged array with a large number of cylinders caused by evanescent mode is presented. Finally, the approximate calculation method of the hydrodynamic coefficients for a large number of rectangular arrays with a given amplitude are summarized. The method is able to efficiently solve the hydrodynamic coefficients of the array with a large number of cylinders, providing an idea to evaluate the performance of such models in engineering.