The present disclosure is directed to a method for assembling a modular rotor blade of a wind turbine. The method includes providing a pre-formed blade root section and a pre-formed blade tip section of the rotor blade. Further, the blade root section includes one or more spar caps extending in a generally span-wise direction. Another step includes providing at least one pre-formed blade segment of the rotor blade. The method also includes mounting the at least one blade segment around the one or more spar caps of the blade root section, wherein the at least one blade segment includes a chord-wise cross-section having multiple joints, wherein at least one joint is located on at least one of a pressure side surface or a suction side surface. In addition, the method also includes joining the blade tip section to at least one of the one or more spar caps or the at least one blade segment.