At a definite temperature,a mesoscopic circuit isn't in a determinate quantum state,but in the mixed state. Using the quantum canonic ensemble theory,the quantum fluctuations of charge and circuit of a mesoscopic LC circuit in the mixed state were studied.It is shown that the quantum fluctuations of a mesoscopic LC circuit depend on not only the circuit component parameters,but also the temperature. The higher the temperature,the more quantum noise the mesoscopic LC exhibits. This method is more easily understood and applied than the TFD method. As the mesoscopic circuit is always at finite temperatures,the result is useful for controlling the quantum fluctuations of mesoscopic circuit.