Kruskal algorithm is a kind of algorithm to find the minimum spanning tree by Joseph Kruskal published in 1956. There are also Prim algorithm and Boruvka algorithm to solve the same problem. These three algorithms are the application of the greedy algorithm. The different place between Kruskal algorithm and Boruvka algorithm is Kruskal algorithm also has effective when they have the same weight of edge in the diagram. There are four steps: 1.To build a new graph G and G has the same node in the original image, but there is no edge. 2. Take all the edges from smallest to largest in the light of weight. 3.At the beginning of alue minimum weight of edge, if this side connecting two nodes in a graph G is not in the same connected components, then add the edges to the graph G. 4.Repeat 3 until all nodes in the graph G are in the same connected components.