Currently, there are no effective results about how much resistance the capsule robot (CR) has to overcome during its start. This paper aims to obtain the critical sliding resistance (CSR), and quantify its influencing factors. Based on Ciarletta's superelasticity model, CSR is analyzed for the "internal force-static friction" CR and magnetically driven CR. Firstly, the resistance is modeled, including the pressure on the CR head and the frictions on the CR middle part and CR head, and a CSR expression is obtained from force equilibrium. Secondly, a experimental platform is built and experiments are performed with in-vitro porcine small intestine. Then, influences of three parameters on CSR are investigated, including ratio between CR's outer diameter and the small intestine's natural inner diameter (R/r), the friction coefficient and the length of the CR middle part. The theoretical value and experimental results for CSR match well with each other. Both the increasing of R/r and friction coefficient will increase the CSR. The friction force on the CR head is less than 1% of the total friction force. The CSR has a linear relationship with the length of CR middle part. The proposed model reflects the influences of various factors, and can accurately predict the CSR. The major factor affecting the resistance lies in the R/r.