The invention discloses a modular rail flaw detection vehicle. The modular rail flaw detection vehicle comprises two detection modules, the two detection modules are connected as a whole through a connection piece; each detection module includes a chassis, an ultrasonic flaw detection wheel and two driving wheels are arranged on the chassis; a rebounding universal ball contacting with the rail isarranged on a limit board at the bottom of the chassis; a chassis cover is detachably connected to the upper part of the chassis; the front end of the chassis cover is connected with a front vision device for detecting the rail surface; both sides of the front portion of the chassis cover are correspondingly connected with a side vision device for detecting whether mounting screws of the rail areloosened. The modular rail flaw detection vehicle also comprises a power supply for the driving wheels on the chassis and a control system inside the chassis cover. The driving wheels, the ultrasonicflaw detection wheel, the front vision device and the side vision device are all electrically connected to the control system. The modular rail flaw detection vehicle can realize autonomous flaw detection, without the need for personnel to follow, and analyze and locate the damage, and the data can be transmitted remotely.