In this paper, a three-dimensional numerical simulation of the merged droplet formation (MDF) in the double T-junction microchannel was performed to investigate the effects of the flow rate of the continuous phase, continuous viscosity and the interfacial tension on the pressure by the level-set method. We divide the droplet formation process into three stages, namely filling, blocking and breakup. We define the time to generate a droplet as the droplet generation period. The characteristic length L/W (L is the length of the droplet, and W is the width of the main channel) decreases as the capillary number increases. The pressure (P-A) at a special position at the junction of the two phases occurs periodically during the droplet formation process, which directly and precisely reflects the mechanism of the MDF in the microchannel. Therefore, we define the time during the pressure fluctuates periodically as the pressure fluctuation period. New insights into the pressure during the MDF in the squeezing time are reported. The results show that the pressure fluctuation period and pressure drop period are the same as the droplet generation period. When the continuous flow rate and viscosity increase, the peak and valley values of P-A and the pressure drop increase, and the time of the pressure cycle decreases. However, when the interfacial tension increases, the peak and theperiod increase and the valley decreases. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.