This paper presents a method of detecting concentrations of solutions based on a complementary split-ring resonator (CSRR) coupled with an open microstrip line resonator. A microfluidic channel is placed on the CSRR. We propose its equivalent capacitively coupled circuit of the sensor and give analytical input impedance and reflection coefficients, which have good agreement with the simulation. The simulated results show for glucose and ethanol water, the sensitivities achieve 18 MHz/l% and 20MHz/l %, respectively, which are excellent comparing with most planar microwave sensors. This verifies that the sensitivity of a sensor based on capacitive coupling can be enhanced.