Solar-powered direct steam generation (DSG) is attractive for power generation and industrial utilization due to the combination of renewable-energy source and clean energy carrier. An improved SIMPLE algorithm ensuring the dual roles of pressure acting on velocity and density fields is developed to realize thermo-hydraulic completely-coupled modeling of a typical DSG loop with transient phase-change and multiple flow-patterns. The excitation-response characteristics of the loop were investigated under various step-variations of direct normal irradiance (DNI), inlet mass fiowrate (m(in)) and inlet temperature (t(in)). Increasing DNI (decreasing m(in)) is found to narrow the preheating-evaporation regions and expand the superheating region, and vice versa. While under step-variations of t(in), the evaporation region almost remains unchanged (about 403 m). The water slides to a lower temperature faster than climbs to a higher one under variations of DN1 (up to 670s vs. 2960s) and m(in) (up to 1184s vs. 4420s), simultaneously the outlet temperature (t(out)) staying a monotonical response-trend. However, under t(in) variations, t(out) holds a higher-order trait. The responses of both pressure and velocity are tightly coupled and always hold higher-order trait. The response time of the total mass in the loop is almost 2.5 to 5.5 times as fast as t(out). (C) 2018 Elsevier Ltd. All rights reserved.