The third-harmonic laser parameters of a laser driver similar to U.S. National Ignition Facility (NIF) in a high laser-flux state, such as space, time and energy, are measured by using the third-harmonic precision diagnostic system of Shenguang II (SGII) upgraded facility. The phase-type periodic structure introduced in the machining process of the quartz components of the terminal optical system is confirmed by improving the near-field resolution. The far-field focusing ability and the near-field transmission modulation characteristics of the terminal optical system in a high laser-flux state are studied. The results show that the phase-type periodic structure is an important reason which can result in an enhancement of third-harmonic near-field modulation.