在AWS Lambda函数中访问另一个AWS账户中的EC2资源，需要进行以下步骤：

1. 建立Role并将其分配给Lambda函数

首先需要在访问EC2实例的AWS账户上建立一个Role，并将其分配给Lambda函数，以授予Lambda函数访问该账户中的EC2实例的权限。

以下是一个例子，Role的名称为“AWS-Lambda-EC2-Access-Role”：

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt0123456789",
            "Effect": "Allow",
            "Action": [
                "ec2:Describe*",
                "ec2:Get*",
                "ec2:RunInstances",
                "ec2:Start*",
                "ec2:Stop*",
                "ec2:TerminateInstances",
                "ec2:CreateTags",
                "ec2:DeleteTags",
                "elasticloadbalancing:Describe*",
                "elasticloadbalancing:RegisterInstancesWithLoadBalancer",
                "elasticloadbalancing:DeregisterInstancesFromLoadBalancer",
                "autoscaling:Describe*",
                "autoscaling:CreateAutoScalingGroup",
                "autoscaling:UpdateAutoScalingGroup",
                "autoscaling:DeleteAutoScalingGroup",
                "autoscaling:CreateLaunchConfiguration",
                "autoscaling:DeleteLaunchConfiguration",
                "autoscaling:CreateOrUpdateTags",
                "cloudwatch:DescribeAlarms",
                "cloudwatch:GetMetricStatistics",
                "cloudwatch:ListMetrics",
                "sns:ListSubscriptions"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}

2. 建立Policy并将其分配给EC2资源

在拥有EC2实例的AWS账户中建立一个Policy，以授予Lambda函数通过Role访问EC2的权限。

以下是一个例子：

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [