To address the critical need for precise detection of early-stage micro-cracks originating from stress concentration around fastener holes in aerospace structures, and to overcome the challenges associated with the poor structural compatibility of traditional piezoelectric ultrasonic sensors and the limited sensitivity of conventional linear guided wave methods, this study proposes a nonlinear guided wave detection method based on a direct-write piezoelectric ultrasonic sensor array, aiming to achieve high-sensitivity identification of micron-scale cracks. The method involves the design and fabrication of a locally enhanced sensor array by integrating the nonlinear effects of guided waves. This array consists of an outer annular excitation element for the fundamental frequency (0.5 MHz) and an inner arc-shaped receiving element optimized for the second harmonic frequency (1 MHz), thereby optimizing the excitation and reception of nonlinear signals. Utilizing the fabrication process of direct-write piezoelectric ultrasonic sensors, an ultrathin and flexible poly vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer piezoelectric ultrasonic sensor array is achieved. This enables in-situ and consistent integration with the surface of the structure under test, effectively eliminating the signal instability issues associated with traditional coupling methods. Building on this foundation, the study introduces a multi-path pulse transmission-reception detection strategy. Frequency-domain analysis and feature extraction of the received ultrasonic signals are performed to calculate the relative nonlinear coefficient, a key parameter characterizing crack-induced nonlinearity. This facilitates crack location identification and establishes a quantitative relationship with crack lengths (2.5,4,6 mm). Experimental results demonstrate that the proposed method effectively captures the evolution trend of nonlinear guided wave features, enabling reliable detection of both the size and propagation direction of early-stage micro-cracks. This work provides a promising new technical pathway for the in-situ, high-precision inspection of critical regions in aerospace structures.