The aim of this work is to analyse the influence of spring length on the simple harmonic motion of a spring-mass system. In particular, we study the effect of changing the spring length on the elastic constant "[kappa]", the angular frequency "[omega]" and the damping factor "[gamma]" of the oscillations. To characterize the behaviour of these variables we worked with a series of springs of seven different lengths, in which the elastic constant was found by means of the spring-elongation measurement and "[omega]" was obtained from the measurement of the oscillation period "T" of a suspended mass. The oscillatory movement was recorded using a force sensor and the "[gamma]" value was determined by the fit of the envelope oscillations. Graphical analysis of the results shows that "[kappa]", "[omega]" and "[gamma]" decrease when the natural spring length increases. This experiment can be performed with equipment normally found in undergraduate physics laboratories. In addition, through graphical analysis students can deduce some relationships between variables that determine the simple harmonic motion behaviour. (Contains 10 figures and 1 table.)