In order to efficiently employ the MVMA-2D, i.e. 2-dimensional occupant movement simulation, the relation between the accuracy of the input data and the results of the simulation is examined. Moreover, the correlation of seatbelt characteristics to the movement and injury level of a dummy constrained by a 3-point seatbelt is investigated quatitatively.Many input data items in analytical simulation, such as geometric conditions and load-deflection conditions are often only rough estimates because of the lack of experimental verification. In particular, since input data on the 3- point seatbelt model to the MVMA-2D has contained many hypotheses due to it's complicated mechanism, the mechanism is clarified experimentally to improve the initial data on the characteristics of the seatbelt and initial dummy location. Using these refined initial conditions, the results of the MVMA-2D simulation come to coincide closely with the experimental results, verifying the reliability of the simulation.This paper describes our method of improving the input data and gives the results of our simulation. Furthermore, utilizing the basic data obtained from the above method, a parameter study was conducted on the load-strain curve of seatbelts, and the relationship of this curve to dummy movement and injury level was derived. These results are also provided in the paper.