The present disclosure provides a flexible substrate of a bendable display device and a manufacturing method thereof, the flexible substrate includes a bendable base layer, which includes a bending region and a non-bending region; the flexible substrate further includes a fiber reinforced layer, which is provided on a surface of the base layer and has different distribution densities in the bending region and the non-bending region. The flexible substrate of a bendable display device provided in the present disclosure can not only protect semiconductor devices on the flexible substrate, and avoid situations such as dislocation of circuits of electronic components, low yield rate and etc., but can also disperse bending stress generated when the flexible substrate is bent, thereby prolonging the bending life of the display device.