China making the first disclosure of its military budget in almost two decades, said yesterday that it will boost defense spending by about 20 percent this year to $12.64 billion.

According to Finance Minister Zhang Jingfu, the increase in military spending was necessitated by the month-long border war with Vietnam during February and March.

Zhang said the overall national budget for 1979 will be the same as that for 1978, or $67.5 billion. This means that the additional money for the military will have to come from other sectors. Presumably, one analyst said, the military increases are at least partially responsible for Peking's decision to trim back its ambitious economic development plan.

Based on the figures disclosed by Zhang, the first on military expenditures since 1960, China will be spending about 19 percent of its budget on defense. Analysts say some military spending, such as weapons development, may be concealed in other areas of the budget, making the actual proportion somewhat higher.

Nevertheless, for comparison purposes, it is estimated that India spent approximately one-third of its 1975 budget on its military establishment and the United States is allocating 23.1 percent of its current budget to the military.

Other than citing the conflict with Vietnam and commenting that "our border defense needs to be strengthened," Zhang gave no reason for the increased military spending. For many years China has been confronted with a hostile Soviet Union across its northern and western frontier.

While China has a modest but growing nuclear missile arsenal, its conventional forces are largely equipped with weapons of the Korean War era, for the most part modifications of tanks, planes and artillery obtained from the Soviet Union at that time.

In one of the more open policy debates in China in recent years, the country's military leaders have argued strenuously that the time had come to modernize the armed forces.

Several Western military specialists, some of whom have visited Chinese military establishments, have argued that the advances in modern weapons technology have rendered Chinese military strategy obsolete.

Peking's military planners, in the event of war with the Soviet Union, had counted on trading territory for time, with the sheer weight of China's manpower providing it with eventual victory.

China's defense expenditures have fluctuated widely over the years. While the variations to some extent reflected changing economic priorities, they have also been the result of internal political struggle within the country.

In the mid-1970s the military's ability to command a sizable share of the national resources was at a particularly low point.

This period coincided with the political ascendency of the radical group identified with Communist Party Chairman Mao Tse-tung.

The radicals, who were quickly purged following Mao's death in 1976, not only kept the regular military on relatively short rations but also made efforts to build up the civilian militia, presumably as a counterweight to the armed forces.