## 1 Introduction  
For conventional closed-hole micro-structured optical fiber for fluid testing, it takes a long time to fill the micro-holes with fluid to be measured. This slow filling rate greatly limits the application of conventional micro-structured optical fibers in fast fluidic detection [1; 2; 3]. The introduction of micro-structured optical fiber with side-opening structure (referred to as opened fiber) greatly reduces the huge contradiction between the slow fluid-filling rate of the traditional microstructured optical fiber fluidic sensing device and the requirement of fast fluidic detection. At the same time, it broadens the application prospect of microstructured optical fiber in the field of fast fluidic sensing [4; 5; 6; 7; 8; 9; 10; 11; 12]. Meanwhile, the opened microstructured optical fiber is also expected to enhance the degree of overlap and the interaction effect between the measured fluid and the optical field within the sensing region, thereby enhancing the sensing performance while achieving rapid detection at the same time [4; 5]. In addition, by optimizing the optical fiber structure, sensing mechanism and signal demodulation methods [6], opened microstructured optical fiber fluidic sensing technology is expected to achieve more sensitive and flexible sensing features. In addition, it enables multi-parameters sensing of fluid properties. Assisted with the labeled biological antibodies or fluorescent substances and other measures [6; 7], the applications of the proposed opened microstructured fiber can be broadened to biomolecular detection. The combination of opened microstructured optical fiber fluid sensing technology and microfluidic chip technology [8; 9] will help improve the integration, portability and sensitivity of microfluidic chip detection systems and the fluid dynamic control of fiber fluidic sensing systems. In addition, the sensing mechanism of surface plasmon resonance (SPR) [10] and fiber grating [11] can further increase the sensitivity and extend the range of application of fluidic sensing. By coating the metal film or the metal particle on the inner surface of an opened optical fiber micro-holes, the surface plasmon resonance (SPR) and the field enhancement can be effectively excited. Based on the upper phenomenon, an opened fiber SPR sensor with extremely high detection sensitivity can be formed. Compared with the prism-based SPR sensor, opened fiber SPR sensor has the advantages of small size, high sensitivity, and low cost. In addition, it is easy to implement the integrated and portable fluidic detection. The types of existing opened fiber used in SPR testing today cover tapered optical fibers [12], D-type optical fibers [13], PBG micro-structured optical fibers [14] and total reflection-type microstructured optical fibers [15]. The sensitivity of the optical fiber SPR sensor was closely related to the factors including the characteristics of the metal, the thickness of the coating and the refractive index range of the detection substance. By optimizing the above parameters, the sensor's sensitivity can be effectively improved. Based on the opened suspended optical fiber structure, a novel optical fiber surface plasmon resonance sensor was designed here. It was characterized in that an air hole in the suspended core optical fiber is opened to the outside, and gold cylinders with the same size and same spacing were arranged along the axial direction of the fiber core inside the air hole cladding. The presence of gold cylinder can stimulate the SPR effect. At the same time, in different spectral bands, this study analyzes the impact of the diameter of the gold cylinder, the interval between the gold cylinder and the refractive index of the fluid under test on the sensitivity. The results show that by optimizing the parameters such as the diameter and spacing of the gold cylinder, its sensitivity \(S\) could be greatly improved.