A new design for a transparent low-pass filter ( LPF) using a flexible polyethylene terephthalate ( PET) material and metallic copper was proposed in this paper. The transparent microstrip filter employs an asymmetric Π-shaped structure with dimensions of 260.000 mm × 13.000 mm × 0.148 mm, which owns a narrow transition band and achieves an insertion loss of less than 1. 0 dB below 0. 26 GHz. Furthermore, the insertion loss of the transparent microstrip filter can exceed 30 dB in the stopband from 1. 02 GHz to 6. 50 GHz. Square holes are added to the copper layer to improve the transparency of the LPF, and the filter performance can be adjusted by changing the square hole area and resolution. Additionally, multiple identical structures are cascaded to increase the insertion loss in the stopband while maintaining the passband. The transparent microstrip LPF has the characteristics of flexibility, high transparency, and significant insertion loss in the stopband, making it suitable for applications that require electromagnetic protection in a clear field of vision.