CHINA'S economy remained unstable and inflation could spiral out of control again if the present economic policy were further relaxed, an Economics Ministry projection said. The ''authoritative Economics Ministry'' assessment came amid fears that China's bold reform blueprint might trigger a fresh round of inflation and economic overheating this year. Projecting from last year's performance, the ministry officials were quoted by the China News Service as saying the economy will rebound swiftly in the next few months - after a brief slow-down of growth in the last few months of 1993. ''It's necessary to implement a tighter fiscal policy so that we will have no choice but to slap more severe measures when the economy overheats again,'' it said. An unregulated money supply, excessive capital investment and inflation in the summer triggered an economic crisis, forcing the central Government to impose a 16-point austerity blueprint to cool off the economy. The austerity programme has virtually come to a halt since November as patriarch Deng Xiaoping orchestrated another wave of high-speed growth. The officials, however, warned that a lot of ''unstable hidden elements'' remained in the Chinese economy. ''The impulse of [increasing] investment is still strong. Many localities are waiting for the delegation of more powers from the central authorities to launch projects.'' The economics ministry warned that enterprises were still plagued by the structural weakness which led to low efficiency. ''If the present fiscal policy which has already been relaxed is further decontrolled, the whole economy is set to surge again. ''It will return to the levels of last July soon. The macro economy will again overheat,'' it warned. The ministry pointed out that the scope and intensity of reforms this year covering fiscal, investment, foreign exchange, housing and prices was unprecedented in the last decade. ''Elements of instability created in the process of reform will also increase. Therefore, the task of macro control and adjustment will be arduous this year,'' it said. The ministry indicated that ''macro control'' should be imposed to stabilise commodity prices and maintain an ''appropriate'' growth rate, which it said should be ''no more than 10 per cent''. The central Government, it said, would continue to maintain ''regular, timely and appropriate fine-tuning'' of the macro economy this year to promptly eliminate the elements of instability. The ministry warned that the relaxation of the money supply from October to December has increased inflationary pressure. Present levels of inflation were high, it said. The recent hefty rise of prices and panic-buying of grain and other staple foods, jewellery and home appliances had reflected the populace's sensitivity to price rises, it said. The Economics Ministry maintained that the price situation should be closely monitored this year following the introduction of tax and wage-system reforms.