High-reliability double-sided ring collector systems have been widely implemented in offshore wind farms (OWFs). It is challenging to achieve a globally optimal network topology and a cable capacity rating for the OWF collector system (CS) simultaneously. This paper proposes an optimal collector system planning (CSP) method for OWF with double-sided ring topology based on bidirectional flow conservation method to minimize cable costs and total power losses. By analyzing the power flow direction after faults, all fault scenarios are summarized into two fault conditions. The bidirectional flow conservation method is developed to reveal the matching mechanism between different cable sequence positions and their optimal ratings, considering the minimal rating requirements. The complex high-dimensional CSP problem, which involves the coupling characteristics of different cable parameters and system power flows, is convexified by equivalent alternative methods into a mixed-integer quadratic programming (MIQP) to guarantee a global optimal solution within feasible computation time, improving the solvability and practicality. The effectiveness of the proposed optimal CSP method has been validated in MATLAB.