Obese people are prone to foot deformities such as flat feet. Foot management programs are important to prevent them. This study investigated the effects of two foot-ankle interventions on balance ability, foot arch, ankle strength, plantar fascia thickness, and foot functions in obese people with pes planus for four weeks. The experiment was designed as a randomized controlled trial. Twenty-four participants who met the inclusion criteria were selected, and they were randomly assigned to either a short foot group (SFG) or proprioceptive neuromuscular facilitation group (PNFG) according to foot-ankle intervention. Two interventions were commenced three times a week for 20 min over four weeks. The tests were conducted at two intervals: pre-intervention and at four weeks. The tests were conducted in the following order: the patient-specific functional scale test (PSFS), an ultrasound of the plantar fascia, the navicular drop test, balance test, and the four-way ankle strength test. Two groups showed significant differences in balance ability, foot arch, ankle strength, plantar fascia thickness, and foot functions between pre-test and post-test ( < 0.05). PNFG had significantly higher dorsiflexor and invertor strength than SFG ( < 0.05). SF and PNF interventions were effective to improve balance ability, foot arch, ankle strength, plantar fascia thickness, and foot functions in obese people with pes planus. Additionally, PNF intervention is more beneficial in increasing the dorsiflexor and invertor strength compared to SF intervention.