The invention discloses a simple micro-channel device for forming a stable annular flow. The simple micro-channel device comprises a base, an inflow guiding and retaining system, an outflow guiding and retaining system, a first inner tube, a second inner tube, an outer tube, a linear center inserting piece and a supporting column. When the linear center inserting piece is made of a lipophilic material, the outer tube is made of a hydrophilic material, and when the linear center inserting piece is made of a hydrophilic material, the outer tube is made of a lipophilic material; and the inflow guiding and retaining system is composed of two first templates with the same shape, structure and dimensions, and the outflow guiding and retaining system is composed of two second templates with the same shape, structure and dimension. The experiments show that the simple micro-channel device is used, and liquid-liquid two-phase fluid forms the stable annular flow in a wider flow ratio range and can be subjected to phase separation immediately when flowing out of a micro-channel. In addition, the micro-channel device has simple preparation process and low cost, and users such as teaching unitswith a large usage amount can conveniently accept the device or prepare the device by themselves.