This paper reveals the inner relationship between the characteristic vectors of Lanchester equations and fire indexes with the help of the property of positive matrixes. By way of examples the paper further shows that the fire indexes are the extension of the notion of power coefficients in Lanchester equations. It is pointed out that the fire indexes are not only related to the weapons，but also depend on the disposition of troops in a whole battle. When some kinds of weapons are destroyed the fire indexes will change with situation. The fire indexes also depend on the principles of weapon assignment. Thus the relationship between fire indexes and tactical principles is set up.