With little in the way of fresh policies from Xi on Monday, all eyes now turn to an expected meeting between him and Trump at the G20 summit in Argentina at the end of the month.
China would "foster a world-class business environment" and its doors will open "ever wider", he said.
Ma, who's planning on retiring next year, has also said the global business community is opposed to the trade war.
Xi has said the import expo proves China is willing to reduce its foreign trade surpluses.
US President Donald Trump has railed against China for what he sees as intellectual property theft, entry barriers to US business and a gaping trade deficit.
Decrying "protectionism", "isolationism" and confrontation, Mr Xi said countries should fix their own houses before targeting others.
"They should not hold a flashlight in hand, doing nothing but highlighting the weaknesses of others and not their own", he added.
"It is our honest commitment to open the Chinese market", Xi said in an address opening the event in Shanghai.
Beijing has touted the first annual China International Import Expo as a sign of its willingness to open its markets despite mounting criticism to the contrary and the worsening trade war with Washington, which has seen both sides impose punitive tariffs on hundreds of billions of dollars' worth of goods.
About 180 USA companies are sending representatives, including big names such as Alphabet Inc's Google, Boeing Co, Caterpillar Inc, Facebook Inc, General Motors Co, Honeywell International Inc, Microsoft Corp, Tesla Inc and Qualcomm Inc.
"It seems like there were good statements and good headlines, but what we want are concrete actions and a concrete timetable of reform", said Carlo Diego D'Andrea, the Shanghai chairman for the European chamber. But that's also "the greatest opportunity for the world".
The Nov 5-10 China International Import Expo (CIIE), brings thousands of foreign companies together with Chinese buyers in a bid to demonstrate the importing potential of the world's second-biggest economy.
Presidents or prime ministers from 17 countries were set to attend the expo, ranging from Russian Federation and Pakistan to the Cook Islands, though none from major Western nations.
"It is important for us therefore to correct the existing trade imbalance to enable a fair share of the benefits of trade", he said. He said China would continue to promote globalization and pledged to boost domestic consumption, strengthen intellectual property enforcement and advance trade talks with Europe, Japan and South Korea.
China ranks 59th out of the 62 countries evaluated by the Organization for Economic Cooperation and Development in terms of openness to foreign direct investment.
Uhuru has been one of the chief guests at the China International Import Expo, the first such event in China that has attracted participants from over 130 countries, among them Kenyan horticultural farmers and traders.
About 3,600 companies from more than 170 nations, regions and global groups are taking part in the country's first national-level expo for imports, the Chinese government said.