Although microfluidic devices have made remarkable strides in blood cell separation, there is still a need for further development and improvement in this area. Herein, we present a novel ultralow aspect ratio (H/W = 1:36) spiral channel microfluidic device with ordered micro-obstacles for sheathless and flow-rate-insensitive blood cell separation. By introducing ordered micro-obstacles into the spiral microchannels, reduced magnitude fluctuations in secondary flow across different loops can be obtained through geometric confinement. As a result, the unique Dean-like secondary flow can effectively enhance the separation efficiency of particles in different sizes ranging from 3 to 15 μm. Compared to most existing microfluidic devices, our system offers several advantages of easy manufacturing, convenient operation, long-term stability, highly efficient performance (up to 99.70% rejection efficiency, including platelets), and most importantly, insensitivity to cell sizes as well as flow rates (allowing for efficient separation of different-sized blood cells in a wide flow rate from 1.00 to 2.50 mL/min). The unique characteristics, such as ultralow aspect ratio, sequential micro-obstacles, and controlled secondary flow, make our device a promising solution for practical plasma extraction in biomedical research and clinical applications.