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The evolution of shock waves produced by 7 ns laser pulses in air is investigated by time-resolved shadowgraph.A nodular structure of the shock wave is observed. It is found that the origin of the structure is the multi-longitudinalmicrofocus caused by the astigmatism of the laser beam. The spherical shock waves formed by each microfocus expand gradually and collide with each other, resulting in the nodular structure of the shock wave.