This paper aims to develop a miniaturized ultrasound transducer for intravascular thrombolysis, which requires a transducer with high mechanical index (> 0.3) and a long focal distance (> 2.0 mm). To meet this need, a composite of a multi-pillar piezo stack (MPPS) transducer was designed and fabricated by using PZT-4, conductive epoxy, soft elastomer, and a metallic concave lens. The simulation results showed the predominant extensional mode at around 0.92 MHz. The −6 dB focal distance was elongated by 150% from that of the common single-pillar piezo stack (SPPS) transducer with the same aperture. The acoustic pressure was improved by 8.2 dB at the focal zone. Next, the experimental results showed the peak-to-negative pressure of 1.92 MPa under 120 Vpp input, which was sufficient to cause the inertial cavitation of microbubble contrast agents. The −6 dB focal distance reached up to 3.4 mm which was a significant improvement compared to the conventional SPPS transducer. The developed intravascular transducer will be employed for the microbubble-mediated ultrasound thrombolysis to reduce the treatment time.