The continued depletion of fossil fuels compels the exploitation of biomass as alternative. Herein, coconut palm, as a common tropical biomass, was first used to prepare polyether polyols via liquefaction method. The obtained polyether polyols were employed to fabricate rigid polyurethane foam (RPUF), which is widely applied in the industry. Moreover, the prepared RPUF was further modified by biomass waste including coconut shell (CS), coconut palm (CP), and seaweed (SE), with the contents of 5–15 wt%. The results showed that the addition of fillers can decrease the apparent densities and increase the compressive strength of the foams. The foam modified with 15 wt% CS exhibits the highest compressive strength with a value of 355 kPa and normalized value of 290 kPa among the prepared RPUF. In addition, an appropriate content of fillers can slightly increase the thermal stability of RPUF. The combustion performance test indicates that the introduction of fillers slightly increases the oxygen index value of coconut-based polyurethane, which produces less smoke than unmodified foam. Compared with the HRR of unmodified foam, the HRR of RPUF modified with CP are higher while those modified with SE are smaller. The foams modified with SE exhibit the optimal flame-retardant property. This work provides a novel and sustainable route to prepare and modify bio-based RPUF, which can furnish a clue to prepare bio-based RPUF with desirable performance.