The paper reviews the whole project and introduces the casing string structure of Riserless Drilling. Then it establishes the shallow Riserless Drilling longitudinal vibration model based on vibration theory, soil mechanics, fluid mechanics. We assume the soil layers around the casing are homogeneous, isotropic and saturated cohesive soil, and the earthquake force is harmonic excitation force. And the harmonic excitation force is at the bottom of the soil layers. Meanwhile, it also considers flow friction of drilling fluid in well bore. Based on this, this paper analysis the longitude forced vibration load of casing string by Riserless Drilling and gets its results. At the same time, this paper simulates the results through Abaqus software and compares it to the calculation results. And the results show that the earthquake degree, and the depth of casing is the main factors of longitudinal vibration of casing string of Riserless Drilling while the rate of drilling fluid has little effect on the vibration.