The invention provides a transmission shaft assembly and a detection device for a mobile phone shell. The transmission shaft assembly comprises a first transmission shaft, one end of which is in a conical shape; a positioning block, which is connected with the other end of the first transmission shaft and located below a mobile phone shell to support the mobile phone shell; and an auxiliary block,one end of which is connected with the side wall of the positioning block, and the other end of which extends away from the positioning block and protrudes from the outer side wall of the mobile phone shell. In the technical scheme proposed by the invention, the positioning block is connected with the first transmission shaft, the movement of the positioning block is driven by the movement of thefirst transmission shaft so as to further drive the movement of the auxiliary block connected with the positioning block, the positioning block is located on the inner side wall of the mobile phone shell, one end of the auxiliary block protrudes from the outer side wall of the mobile phone shell, and thus, dimensional parameters of the inner side wall of the mobile phone shell can be measured.