Magnetic mobile microrobots navigating biofluids with both upstream and downstream locomotion provide a promising solution to targeted drug delivery for precision medicine. However, the biofluid environment in blood vessels is complicated due to variations in flow rate and direction. It is still unknown how to make magnetic microrobots resist the variable flow rate in biofluids with both upstream and downstream locomotion. Herein, magnetic microrobots with various shapes and sizes have been controlled to navigate diverse biofluids under different flow rates and directions. Simulation and experimental studies have been conducted to analyze the influences of microrobot size and shape on translational velocity in confined microchannels filled with biofluids. A strategy is proposed to choose the optimized parameters of rotating magnetic field actuation for precise delivery of microrobots in a microfluidic chip, which contains a complex biofluid environment with variable flow rate and direction. The results are validated using various microrobots navigating the microfluidic chip and the yolks of zebrafish larvae in vivo. This work provides a guideline for selecting desirable microrobot dimensions and magnetic field actuation parameters for controllable navigation of magnetic mobile microrobots in complex biofluid flows.