The Constitutional Court ruled on Wednesday that the bill on the election of MPs, which governs the new party-list system, did not contradict the constitution.
The nine judges voted unanimously that the bill was in compliance with the charter.
The decision means the introduction of a divisor of 100 to calculate the number of list MPs at the next election remains intact.
Rawee Matchamadol, leader of the New Palang Dharma Party, petitioned the court through Parliament President Chuan Leekpai, challenging the validity of the bill, after parliament decided to use a divisor of 100.
New Palang Dharma and other small parties would have difficulty garnering enough votes to win a list seat under this method. They favoured the formula used in the last election, which used 500 to distribute party-list seats.
The two numbers came from different bases. The 100 divisor is from the total number of party-list MPs, while the 500 divisor is the total number of elected MPs in the House. The two formulas use different bases for calculation of the number of list MPs each party would have.
The coming general election will be for 400 constituency MPs and 100 party-list MPs. The 2019 polls installed 350 electorate MPs and 150 party-listers.