The ambience has a critical effect on the characteristic of bead formed by fuse breaking of the electric copper wire in fire. In order to study the influence of oxygen concentration in surroundings on the characteristic of bead formed by fuse breaking, firstly, the oxygen concentration of typical things such as wood, paper, foam, rubber and plastic etc when they were burning was measured. The extreme conditions of oxygen concentration of typical things were ascertained when they were burning. Accordingly the oxygen concentration of simulated environment (100% N2, 10% O2+90% N2, and 20% O2+80% N2) was determined. Secondly, the in-depth composition of beads formed by fuse breaking of the electric copper wire in different circumstances was studied by AES. The relationship is almost linearity between the average oxygen concentration and the ambient oxygen concentration. Consequently, from the measured oxygen concentration, the authors can deduce the ambient oxygen concentration and the fire cause.