The efficient structure for optical fiber based surface Plasmon resonance sensor has been investigated. The sensor structure is based on removing the cladding of a multimode optical fiber and replacing it with thin film of gold. The incident light inside the fiber will excite surface plasmons which will be resulted in the absorption in specific wavelengths. The absorption behavior is dependent on the dielectric constants at the vicinity of the metallic thin film. Based on the amount of removed cladding and deposition regions on the core structure of the fiber, different configurations can be considered. Based on the computational analysis, we have studied performance of sensor for three different configurations. Our method of study is based on solving wave equation in different sensor geometry and calculating absorption for different available modes using Finite Element Method in COMSOL environment. The results of our study show that, the optimum configuration of sensor considering wavelength sensitivity is for a fiber which half of core and cladding has been removed and the gold thin film is deposited on its surface.