It is very important to research and analyze one of Knapsack Cryptosystems in which the density of knapsack is greater than 0.9408. For the new knapsack public-key algorithm which is based on linear indeterminate equation in two variables, public key and ciphertext are used to construct a lattice to attack the scheme and the NTL library to verify the efficiency of the lattice attack algorithm, which finally illustrates the effectiveness of the attack method. It then proves the new knapsack public-key cryptosystem is insecure.