In this study, a method to fabricate a polymer-based flexible pulsating heat pipe (PHP) as a heat spreader for application to flexible devices was suggested and the thermal performance of the flexible PHP was investigated. Low-density polyethylene (LDPE), which is used as the base material was cut into a channel shape by a femtosecond laser and then covered with a multilayer Al-foil film using a heat sealing method. To block the permeation through the flange side, indium, which has high flexibility, was coated. This indium coating was protected by UV-curing resin to avoid damage from bending. The experiment was conducted according to various input powers at various bending angles; $0^\circ$, $30^\circ$, $60^\circ$, $90^\circ$. The thermal resistance decreases as the input power increases, as in a general PHP characteristic. At 12W, the minimum thermal resistance is 3.2K/W and this is 17% lower than the copper reference sample, which was covered with the same film. The thermal resistance slightly increases as the bending angle increases. When the bending angle is $90^\circ$, the thermal resistance increases 7.5% on average. To verify the reliability of the fabricated PHP, the thermal resistance was compared over time in the accelerated environment which is consist of 1bar of air and 1bar of $CO_2$. The thermal performance was maintained over 4.9days in the accelerated environment corresponding with 3.1 months in SATP (standard ambient temperature and pressure) conditions. Therefore, the indium shield can effectively block the permeation of non condensable gases (NCGs) through the flange side. The flexible PHP fabricated in this study is much lighter and has higher effective thermal conductivity than copper sample while providing high flexibility and thus can be used as a flexible heat spreader in flexible devices.