The collision of a keyed hash function based on chaotic map is pointed out. Its principle is analyzed in theory. The definition of the nonsingularity is presented based on analyzing digital discrete chaotic sequence. The necessary and sufficient conditions for the nonsingularity is deduced. The period of digital discrete chaotic sequence with variable parameter is discussed. The result shows that the singulartiy of chaotic sequence leads to the collision of the hash function. So the digital method of chaotic map must be chosen carefully to ensure the nonsingularity of chaotic sequence.