This study presents the signal processing techniques applied on a 9.4/18.8 GHz harmonic radar, which is used to investigate behaviours of small insects such as bees, beetles and butterflies. It is still a challenge to achieve high accuracy and long detection range simultaneously in a harmonic pulse radar system. The authors combine the spread spectrum technology with the classic pulse harmonic radar system to overcome the dilemma between accuracy and detection range. The processing gain of the pseudo-random code from the spread spectrum technology is used to increase the sensitivity and makes the system able to detect the signal with the weak power strength below the system noise level. To eliminate the effect caused by the local leakage, the signal-processing method provided to cancel the leakage through applying the symmetric property of the autocorrelation function of the pseudo-random code. In the field tests, the proposed system achieves a 60 m detection range within 1 m ranging accuracy by using 1.75 W transmitting power corresponding to 40.430 dBW equivalent-isotropically radiated power (EIRP). It is estimated to extend to at least 900 m detection range by using a 3 kW transmitting power corresponding to 72.771 dBW EIRP.