Due to the advantages of high sensitivity, reliable performance and high measurement accuracy, the thermal microflow sensors are widely used in various fields, such as environment monitoring systems, biological research, process control chemical engineering, and so on. However, the limited range of such sensor impedes the extension of application fields. In this article, a new double-layer and two-channel thermal microflow sensor is proposed for wider measure range. The temperature-difference output is from the sensor in mainchannel at low flowrate, from that in side-channel at high flowrate. To improve the performance of microflow sensor, simulations are conducted to obtain the preferential parameters, such as the size and position of inlet for side-channel, as well as the height and width of main-channel and side-channel. The measure range of improved microflow sensor is 0~ 145 m/ s when the sensitivity being greater than 0. 03 K/ (m·s -1 ). The switching threshold of inlet velocity for temperature-difference output from the main-channel to side-channel is 63 m/ s. Finally, the viscous dissipation of in the microflow sensor is analyzed and found, the viscous dissipation at the front-end of the upstream detector is the main cause of the failure of microflow sensor. This can provide an effective method for improving the performance of the thermal microflow sensor.